kernel: Remove darwin support

Darwin kernel build is not supported for >=5.10
and non-standard clang versions.
Another side effect is that it causes LD_LIBRARY_PATH
to get overwritten, which breaks certain configurations.
Let's just get rid of it since AOSP darwin build is no longer supported.

Change-Id: I9ac642996b90937bd9c52138fb06bf1eff901c8b