memdebug: revert setting gcc 11+ deallocator attribute

memdebug: revert setting gcc 11+ deallocator attribute

These attributes were causing unexplained warnings while playing with
PR #16738: In `CURLDEBUG` builds with mingw, gcc (14.2.0), and `-O3`,
while building `libcurlu`. `-O3` is required. May be related to having
the `CURLDEBUG` allocators in the same source file as their callers
(unity mode). PR #16738 moves `memdebug.c` into the main unity unit.
Unclear why it doesn't affect `libcurl`.

E.g. CI job `mingw, CM ucrt-x86_64 schannel R TrackMemory` CI job:
https://github.com/curl/curl/actions/runs/13888662354/job/38856868429

It also reproduces in an isolated example.

Drop this attribute till we learn more about it.

Ref: https://github.com/curl/curl/pull/16737#issuecomment-2727681306
Partial revert of d5b403074ed2b28e41e0c17dcd436252d38adfa3 #16737

Closes #16740