Remove Windows-specific minidump plugin

With the cross-platform minidump plugin working, the Windows-specific one is no longer needed. This eliminates the unnecessary code.

This does not eliminate the Windows-specific tests, as they hit a few cases the general tests don't. (The Windows-specific tests are currently passing.) I'll look into a separate patch to make sure we're not doing too much duplicate testing.

After that I might do a little re-org in the Windows plugin, as there was some factoring there (Common & Live) that probably isn't necessary anymore.

Differential Revision: https://reviews.llvm.org/D26697

llvm-svn: 287113