[ARM] Remove unused declaration RemapAddSubWithFlags

The corresponding function definition was removed by:

  commit e891654a
  Author: Evan Cheng <evan.cheng@apple.com>
  Date:   Tue Aug 30 01:34:54 2011 +0000

While we are at it, this patch removes ARMPCLabelIndex, for which the
host compiler issues an unused variable warning.