mingw: drop the rest of compat/nedmalloc/malloc.c.h

The third and final chunk removes the remainder of malloc.c.h, from
the "System allocation" banner to the end of the file, and the file
itself. With this commit the compat/nedmalloc/ directory is fully
retired from the tree.

Assisted-by: Opus 4.7
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho