remove assetsDir backwards compatibility (#3341)

the `assetsDir` option for the `@Component` decorator was deprecated
some time ago in favor of the `assetsDirs` option, but we have retained
backwards-compatibility with the old option nonetheless. this commit
removes that backwards compatibility.

STENCIL-410: Remove Backwards Compatibility for assetDir Field on @component

BREAKING CHANGE: setting `assetsDir` on a component will no longer work.
Users should migrate to `assetsDirs` instead.

src/compiler/transformers/decorators-to-static/component-decorator.ts

@@ -1,4 +1,4 @@
-import { augmentDiagnosticWithNode, buildError, buildWarn, isString, validateComponentTag } from '@utils';
+import { augmentDiagnosticWithNode, buildError, validateComponentTag } from '@utils';
 import ts from 'typescript';
 
 import type * as d from '../../../declarations';
@@ -41,13 +41,6 @@ export const componentDecoratorToStatic = (
 
   const assetsDirs = componentOptions.assetsDirs || [];
 
-  if (isString((componentOptions as any).assetsDir)) {
-    assetsDirs.push((componentOptions as any).assetsDir);
-    const warn = buildWarn(diagnostics);
-    warn.messageText = `@Component option "assetsDir" should be renamed to "assetsDirs" and the value should be an array of strings.`;
-    augmentDiagnosticWithNode(warn, componentDecorator);
-  }
-
   if (assetsDirs.length > 0) {
     newMembers.push(createStaticGetter('assetsDirs', convertValueToLiteral(assetsDirs)));
   }