Remove LinuxClangCppflags

This commit removes LinuxClangCppflags.  Adding these `-isystem` flags
results in build failures when we build a module with `libc++_host`.
Furthermore, these headers are not being used anymore.

Bug: 111579848
Test: make checkbuild  # on aosp and internal
Change-Id: Ideca268431d9d60d383e3e5b7bb3f73ece416bd2