universal-access: Remove the mnemonic-activate handler

It's wrong because the label's parent isn't a GtkListBoxRow which
causes a warning.

In any case, we don't need it since GtkLabel already does the work of
finding a suitable parent widget to activate on mnemonic-activate
which ends up causing GtkListBox to fire its row-activated signal that
we already handle.

https://bugzilla.gnome.org/show_bug.cgi?id=708283