avutil/mem: remove av_realloc / av_malloc incompatibility warning

memalign() is not guranteed to be compatible with free() or realloc()
and for platforms in this category we have --enable-memalign-hack
(which should be enabled automatically if such system is detected)
Trying to somehow half support systems that can free() memalign memory
but not reallocate it seems not worth the amount of work needed to
keep 2 then incompatible allocation systems and ensure their
seperation. That is unless this would affect a major platform
on which we want to avoid the memalign hack code

The warnings also cause wasted time and effort as people try
to maintain a separation that does not exist currently.

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
