Remove AsmPrinter::needsRelocationsForDwarfStringPool() since it's
just calling into MAI and is only abstracting for a single interface that
we actually need to check in multiple places.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@198092 91177308-0d34-0410-b5e6-96231b3b80d8