[libclang] Remove LIBCLANG_INCLUDE_CLANG_TOOLS_EXTRA

LIBCLANG_INCLUDE_CLANG_TOOLS_EXTRA causes clang-tools-extra tools
to be included in libclang, which caused a dependency cycle. The option
has been off by default for two releases now, and (based on a web search
and mailing list feedback) nobody seems to turn it on. Remove it, like
planned on https://reviews.llvm.org/D79599

Differential Revision: https://reviews.llvm.org/D97693