Make malloc hooks symbols compat-only so that new applications cannot
link against them and remove the declarations from the API.  Also
remove the unused malloc-hooks.h.

Finally, mark all symbols in libc_malloc_debug.so as compat so that
the library cannot be linked against.

Add a note about the deprecation in NEWS.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>
Tested-by: Carlos O'Donell <carlos@redhat.com>