[clang-tidy] Remove the old ClangTidyCheck::registerPPCallbacks method

Summary:
All in-tree clang-tidy checks have been migrated to the new
ClangTidyCheck::registerPPCallbacks method. Time to drop the old one.

Reviewers: sammccall, hokein

Reviewed By: hokein

Subscribers: xazax.hun, jkorous, arphaman, kadircet, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D60197

llvm-svn: 357582