fix tsc build error for @actions/http-client (#402)

Author: jk2K

tsconfig.json

@@ -2,9 +2,6 @@
   "compilerOptions": {
     "target": "es6",                          /* Specify ECMAScript target version: 'ES3' (default), 'ES5', 'ES2015', 'ES2016', 'ES2017', 'ES2018', 'ES2019' or 'ESNEXT'. */
     "module": "commonjs",                     /* Specify module code generation: 'none', 'commonjs', 'amd', 'system', 'umd', 'es2015', or 'ESNext'. */
-    "lib": [
-      "es6"
-    ],
     "outDir": "./lib",                        /* Redirect output structure to the directory. */
     "rootDir": "./src",                       /* Specify the root directory of input files. Use to control the output directory structure with --outDir. */
     "sourceMap": true,