Remove dead function user_cache_directory()

It's been unused since it was added almost 3 years ago in
https://reviews.llvm.org/D13801

Motivated by https://reviews.llvm.org/rL342002 since it removes one of the
functions keeping a ref to SHGetKnownFolderPath.

Differential Revision: https://reviews.llvm.org/D52184

llvm-svn: 342485