refactor: deprecate protractor elementExplorer

Protractor `elementExplorer` debugger and element explorer cannot be used for Node.js 8+ since it relied on `_debugger` module.

In protractor version 5, this resulted in the below error:
```
** Angular Live Development Server is listening on localhost:4200, open your browser on http://localhost:4200/ **
: Compiled successfully.
[10:25:35] I/direct - Using ChromeDriver directly...
[10:25:37] I/protractor -
[10:25:37] I/protractor - ------- Element Explorer -------
[10:25:37] I/protractor - Starting WebDriver debugger in a child process. Element Explorer is still beta, please report issues at github.com/angular/protractor
[10:25:37] I/protractor -
[10:25:37] I/protractor - Type <tab> to see a list of locator strategies.
[10:25:37] I/protractor - Use the `list` helper function to find elements by strategy:
[10:25:37] I/protractor -   e.g., list(by.binding('')) gets all bindings.
[10:25:37] I/protractor -
***********************************************************
* WARNING: _debugger module not available on Node.js 8    *
* and higher.                                             *
*                                                         *
* Use 'debugger' keyword instead:                       *
* https://goo.gl/MvWqFh                                   *
***********************************************************
/Users/alanagius/cli-repos/demo-several/node_modules/protractor/built/debugger/debuggerCommons.js:14
  throw e;
  ^

Error: Cannot find module '_debugger'
Require stack:
- /Users/alanagius/cli-repos/demo-several/node_modules/protractor/built/debugger/debuggerCommons.js
- /Users/alanagius/cli-repos/demo-several/node_modules/protractor/built/debugger/clients/explorer.js
    at Function.Module._resolveFilename (internal/modules/cjs/loader.js:980:15)
    at Function.Module._load (internal/modules/cjs/loader.js:862:27)
    at Module.require (internal/modules/cjs/loader.js:1042:19)
    at require (internal/modules/cjs/helpers.js:77:18)
    at Object.<anonymous> (/Users/alanagius/cli-repos/demo-several/node_modules/protractor/built/debugger/debuggerCommons.js:3:18)
    at Module._compile (internal/modules/cjs/loader.js:1156:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:1176:10)
    at Module.load (internal/modules/cjs/loader.js:1000:32)
    at Function.Module._load (internal/modules/cjs/loader.js:899:14)
    at Module.require (internal/modules/cjs/loader.js:1042:19) {
  code: 'MODULE_NOT_FOUND',
  requireStack: [
    '/Users/alanagius/cli-repos/demo-several/node_modules/protractor/built/debugger/debuggerCommons.js',
    '/Users/alanagius/cli-repos/demo-several/node_modules/protractor/built/debugger/clients/explorer.js'
  ]
}
```

but in protractor version 7, this logic was removed

packages/angular_devkit/build_angular/src/protractor/index.ts

@@ -26,7 +26,6 @@ interface JasmineNodeOpts {
 
 function runProtractor(root: string, options: ProtractorBuilderOptions): Promise<BuilderOutput> {
   const additionalProtractorConfig: Partial<ProtractorBuilderOptions> & Partial<JasmineNodeOpts> = {
-    elementExplorer: options.elementExplorer,
     baseUrl: options.baseUrl,
     specs: options.specs && options.specs.length ? options.specs : undefined,
     suite: options.suite,

packages/angular_devkit/build_angular/src/protractor/schema.json

@@ -38,7 +38,8 @@
     "elementExplorer": {
       "type": "boolean",
       "description": "Start Protractor's Element Explorer for debugging.",
-      "default": false
+      "default": false,
+      "x-deprecated": "This option has no effect. See: https://github.com/angular/protractor/blob/master/docs/debugging.md#enabled-control-flow for alternative methods."
     },
     "webdriverUpdate": {
       "type": "boolean",

tests/legacy-cli/e2e/tests/basic/e2e.ts

@@ -57,13 +57,6 @@ export default function () {
           },
     `, `allScriptsTimeout: 11000,`
     ))
-    // Should start up Element Explorer
-    .then(() => execAndWaitForOutputToMatch('ng', ['e2e', 'test-project', '--element-explorer'],
-      /Element Explorer/))
-    .then(() => killAllProcesses(), (err: any) => {
-      killAllProcesses();
-      throw err;
-    })
     // Should run side-by-side with `ng serve`
     .then(() => execAndWaitForOutputToMatch('ng', ['serve'],
       /: Compiled successfully./))