[libcxxabi] Cleanup and adapt for r291275. NFC.

+ Now that libcxxabi shares the same threading API as libcxx, a whole
  chunk of code in src/config.h is made redundant (I missed this earlier).

+ r291275 split off the externalized-thread-api libcxx configuration from the
  external-thread-library libcxx configuration. libcxxabi should follow the
  same approach.

llvm-svn: 291440