Selecting the right implementation for comparing typeinfos is not something
that we should configure in the config site. It's specific to the ABI in
use, and it should be detected automatically by the library. It has been
a source of confusion and bugs ever since we've introduced that.
Furthermore, it's really weird to set the comparison implementation
explicitly, yet for that choice not to be honored on arm64/Apple.
This commit removes the ability to explicitly select the implementation
of typeinfo comparison, and instead relies on the existing logic to select
the right one based on the ABI.