Remove Unix specific hack in Encoding.GetEncoding

Remove Unix specific hack in Encoding.GetEncoding

During Hello World bring-up for Unix, we ran into a crash which at the
time I thought was incorrectly due to some missing support in
globalization leading to a first chance exception being thrown.

Instead the problem was that due to a JIT bug, a hashtable that the
Encodings code tried to construct was getting bad data passed to the
constructor which caused an exception to try to be thrown.  The change
I provided caused the hashtable codepath to not be taken and hence
masked the issue.

Now that the JIT bug is fixed, we can remove this workaround. I
verified that HelloWorld still runs on Unix with these changes.