Remove redundant checks against MIN_CACHE_BITS

With the removal of the libmpeg2 bitstream reader, MIN_CACHE_BITS
is always >= 25, so tests against smaller values can be removed.

Signed-off-by: Mans Rullgard <mans@mansr.com>
(cherry picked from commit f162e988aa0dc8df93079b5ebf452ec281ab8793)
