[GlobalISel] Remove the GISelAccessor API.

Its sole purpose was to avoid spreading around ifdefs related to
building global-isel. Since r309990, GlobalISel is not optional anymore,
thus, we can get rid of this mechanism all together.

NFC.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@310115 91177308-0d34-0410-b5e6-96231b3b80d8
10 files changed