configure: prune some more crap

All of the removed lines are hopefully useless and didn't do anything.
configure: simplify arch macros
osdep: remove unused mmap compatibility hacks

Not sure how this worked. Only af_export.c and tvi_v4l2.c were
using mmap, but they didn't include osdep/mmap.h or mmap_anon.h. In
any case, we trust that the target system is sufficiently POSIX
compliant if mmap is actually defined (as checked by configure).