[analyzer] Drop support for GC mode in RetainCountChecker

A lot of code in RetainCountChecker deals with GC mode.
Given that GC mode is deprecated, Apple does not ship runtime for it,
and modern compiler toolchain does not support it, it makes sense to
remove the code dealing with it in order to aid understanding of
RetainCountChecker.

Differential Revision: https://reviews.llvm.org/D50747

llvm-svn: 340091