Remove IgnoreCorLibraryDuplicatedTypes reflection from scripting once CoreFx deals with duplicate types correctly

[filipw]

This is a tech debt tracking issue.

Due to the fact scripting works mainly with implementation assemblies since the reference assemblies are not available, it is possible we end up in situations when error CS0433 is incorrectly reported for Corlib. (#784)

At the moment this is solved via reflection (see PR #785).
However this can be cleaned up in the long term, when the underlying issue is resolved in CoreFx (https://github.com/dotnet/corefx/issues/5540 - the duplicates will be marked internal) - obviously we don't want to carry reflection based code.

[filipw]

Flagging as blocked until the solution is available in CoreFx.