Remove PROTOTYPE comments in the compiler (#46172)

They are recorded in #46171

src/Compilers/CSharp/Portable/Compilation/CSharpSemanticModel.cs

@@ -2217,8 +2217,6 @@ private static bool IsUserDefinedTrueOrFalse(BoundUnaryOperator @operator)
                     type = cond.NaturalTypeOpt;
                     convertedType = cond.Type;
                     convertedNullability = nullability;
-                    // PROTOTYPE(ngafter): Should we preserve the set of conversions?
-                    // By now they have been pushed into the subexpressions.
                     conversion = Conversion.MakeConditionalExpression(ImmutableArray<Conversion>.Empty);
                 }
                 else

src/Compilers/CSharp/Test/IOperation/IOperation/IOperationTests_IThrowOperation.cs

@@ -2200,7 +2200,6 @@ void F(bool x, bool y, System.Exception ex1, System.Exception ex2)
 ";
             VerifyOperationTreeForTest<BlockSyntax>(compilation, expectedOperationTree);
 
-            // PROTOTYPE(ngafter): Investigate the tree in block B4 (syntax for the operand of the conversion)
             string expectedGraph = @"
 Block[B0] - Entry
     Statements (0)
@@ -2293,7 +2292,6 @@ void F(bool x, bool y, System.Exception ex1, System.Exception ex2)
 ";
             VerifyOperationTreeForTest<BlockSyntax>(compilation, expectedOperationTree);
 
-            // PROTOTYPE(ngafter): Investigate the tree in block B4.  The conversion should be an identity conversion on the whole conditional.
             string expectedGraph = @"
 Block[B0] - Entry
     Statements (0)