This reverts commit eab4929a.

Oups it would seem gtk_accel_map_change_entry() could return FALSE even
when the expected shortcut is correctly set (my guess is that it was
already the same shortcut, so indeed no "change" happened, though it is
not a failure either; yet I haven't checked if that is the actual
reason).
Let's just revert this. It's not always a good thing to be too thorough!
Sorry for this!