Remove some unused private fields from the AArch64MCCodeEmitter. These
fields were only ever set in the constructor. The create method retains
its consistent interface so that these bits can be re-threaded through
the emitter if they're ever needed.

This was found by the -Wunused-private-field Clang warning.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175482 91177308-0d34-0410-b5e6-96231b3b80d8