CLEANUP: stats: drop stats.h / stats-t.h where not needed

CLEANUP: stats: drop stats.h / stats-t.h where not needed

A number of C files include stats.h or stats-t.h, many of which were
just to access the counters. Now those which really need counters rely
on counters.h or counters-t.h, which already reduces the amount of
preprocessed code to be built (~3000 lines or about 0.05%).