Details:
- Removed the commented-out #define BLIS_NUM_ARCHS in bli_type_defs.h
and its associated (now outdated) comments. BLIS_NUM_ARCHS has been
part of the arch_t enum for some time now, and so this change is
mostly about removing any opportunity for confusion for people who
may be reading the code. Thanks to Minh Quan Ho for leading me to
cleanup.