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

 - `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`