fix(language-service): Remove completions for let and of in ngFor (#34434)

`let` and `of` should be considered reserved keywords in template syntax
and thus should not be part of the autocomplete suggestions.

For reference, TypeScript does not provide such completions.

This commit removes these results and cleans up the code.

PR Close #34434

packages/language-service/test/project/app/parsing-cases.ts

@@ -111,12 +111,6 @@ interface Person {
   street: string;
 }
 
-@Component({
-  template: '<div *ngFor="~{for-empty}"></div>',
-})
-export class ForOfEmpty {
-}
-
 @Component({
   template: '<div *ngFor="let ~{for-let-empty}"></div>',
 })
@@ -131,7 +125,7 @@ export class ForLetIEqual {
 
 @Component({
   template: `
-    <div *ngFor="~{for-let}let ~{for-person}person ~{for-of}of ~{for-people}people">
+    <div *ngFor="let ~{for-person}person of ~{for-people}people">
       <span>Name: {{~{for-interp-person}person.~{for-interp-name}name}}</span>
       <span>Age: {{person.~{for-interp-age}age}}</span>
     </div>`,