[RDY] Remove memory related dead code and replace vim_realloc by realloc

 - Delete dead code related to `MEM_PROFILE`
> Code around `#ifdef MEM_PROFILE` was used to profile vim's memory
> consumption. 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`