Remove a broken win32 locale function redirection

One can't replace vsscanf(_l) with a sscanf(_l) that doesn't
take a va_list.

This has been untouched since it was added in SVN r140728, so
apparently it hasn't been used since. One reason for this mistake
originally might have been that there was no _vsscanf_l until MSVC
2015.

Since it's unused, just remove this define.

Differential Revision: https://reviews.llvm.org/D40323

llvm-svn: 318810