Remove obsolete function TargetRegistry::getClosestTargetForJIT()

This was kept around "for compatibility through 2.6" in 2009 and is not used or
tested.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212095 91177308-0d34-0410-b5e6-96231b3b80d8