[Driver] MinGW: Remove custom linker detection

This is a re-apply of rL313082 which was reverted in rL313088

In rL289668 the ability to specify the default linker at compile time
was added but because the MinGW driver used custom detection we could
not take advantage of this new CMAKE flag CLANG_DEFAULT_LINKER.

rL289668 added no test cases and the mingw driver was either overlooked
or purposefully skipped because it has some custom linker tests
Removing them here because they are covered by the generic case.

Reviewers: rnk

Differntial Revision: https://reviews.llvm.org/D37727

llvm-svn: 317397