Remove fakethr.h and eliminate all references to it and FAKE_THREADS

fakethr.h and FAKE_THREADS were for a "green" threads implementation of
5005threads. 5005threads itself is long gone, and it's not clear that
-DFAKE_THREADS *ever* built correctly. Certainly it did not work for the
5.005 release, and it did not work at the time of the commits for the initial
checkin. The closest that it seems to have been to working is around commit
c6ee37c52f2ca9e5 (Dec 1997), where the headers no longer contained errors,
but perl.c failed to compile.