Remove not implemented exception (microsoft#1982)

* Remove stale reference

* Don't suppress LHS diagnostics on augmented assign

* Revert "Don't suppress LHS diagnostics on augmented assign"

This reverts commit 6109ac7.

* Escape [ and ]

* PR feedback

* Remove exception

Author: Mikhail Arkhipov

src/Analysis/Ast/Impl/Types/Collections/PythonCollectionType.cs

@@ -67,7 +67,7 @@ public override IMember Index(IPythonInstance instance, IArgumentSet args)
             => (instance as IPythonCollection)?.Index(args) ?? UnknownType;
 
         public IPythonType CreateSpecificType(IArgumentSet typeArguments) {
-            throw new NotImplementedException();
+            return this;
         }
 
         #endregion

src/Parsing/Impl/Ast/TypeAnnotation.cs

@@ -27,9 +27,6 @@ public TypeAnnotation(PythonLanguageVersion version, Expression expr) {
             Expression = expr ?? throw new ArgumentNullException(nameof(expr));
         }
 
-        public static TypeAnnotation FromType<T>(TypeAnnotationConverter<T> converter, T type) where T : class
-            => throw new NotImplementedException();
-
         public PythonLanguageVersion LanguageVersion { get; }
         public Expression Expression { get; }