[Analysis] Remove unused class LegacyAARGetter

The last use was removed by:

  commit fa6ea7a4
  Author: Arthur Eubanks <aeubanks@google.com>
  Date:   Mon Mar 20 11:18:35 2023 -0700

Once we remove it, createLegacyPMAAResults and createLegacyPMAAResults
become unused, so this patch removes them as well.

Differential Revision: https://reviews.llvm.org/D151787