[System.Core] Bring .NET implementation of ReaderWriterLockSlim.
[System.Core] Removed no longer used files.
System.Core is now using .NET implementations of LockRecursionPolicy and ReaderWriterLockSlim.
Merge pull request #1436 from esdrubal/readerwriterlockslim

[System.Core] Bring .NET implementation of ReaderWriterLockSlim.