Remove flags from ClangLibToolingUnknownCflags

This commit removes flags from ClangLibToolingUnknownCflags.  These
flags were added to workaround an old version of clang in
`external/clang`.  Since we have updated to clang-r339409b, we don't
need these workaround anymore.

Bug: 117764794
Test: development/vndk/tools/header-checker/utils/create_reference_dumps.py
Change-Id: Ifbeebf05df5c08ee3bb7d489f39250d8fd5f5acf