remove Cmake option for LLVM_DISABLE_ABI_BREAKING_CHECKS_ENFORCING

This is a follow-up to my change in r295090, which added support for
disabling these checks selectively based on setting the preprocessor
macro without relying on the Cmake setting. Swift has moved over to use
that approach, so we can clean up here and remove the Cmake setting.

https://reviews.llvm.org/D30578

llvm-svn: 297109