[libcxx] Link to fewer MSVC CRT libraries

The library msvcrt.lib pulls in ucrt.lib and vcruntime.lib anyway,
there's no need to manually link against the individual dependencies.

This matches how the tests link against libraries - they only link
against msvcrt and msvcprt, not directly against ucrt and vcruntime.

Differential Revision: https://reviews.llvm.org/D155555