Remove MEM_PROFILE related code

Code around `#ifdef MEM_PROFILE` was used to profile vim's memory
comsumption. It's very likely broken as new code is using malloc() and free()
directly.

In this day and age, valgrind can solve in a much reliable way what
this code was trying to do.
HAVE_AVAIL_MEM is always undefined. Remove tests and dead code

Defining HAVE_AVAIL_MEM doesn't even build. The code tries to call
`mch_avail_mem` which is not defined.
Use realloc instead of vim_realloc