Remove completely irrelevant code from USR generation.

Thanks to Richard Smith for pointing this out.  This code stopped
serving its purpose during r103212 in a refactoring.  My initial
fix was to add back the logic to abort the USR generation for
InternalLinkage, but enough tests broke suspiciously that I fear
that USR generation for cursors with InternalLinkage is now expected
by some clients (where it wasn't the case when the refactoring
took place).  I don't own this code anymore and have not looked
at it for some time, but clearly this code is dead and can be removed
pending further review on the proper logic here.

llvm-svn: 167442