MINOR: activity/memprofile: offer a function to unregister stale info

MINOR: activity/memprofile: offer a function to unregister stale info

There's actually a problem with memprofiles: the pool pointer is stored
in ->info but some pools are replaced during startup, such as the trash
pool, leaving a dangling pointer there.

Let's complete the API with a new function memprof_remove_stale_info()
that will remove all stale references to this info pointer. It's also
present when USE_MEMORY_PROFILING is not set so as to ease the job on
callers.

(cherry picked from commit 859341c1ec583c586ef36db0b63cd84f3843bfab)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>