loginDialog: Rely on default button for activation

loginDialog: Rely on default button for activation

Currently the default action is performed twice when pressing Return
in the login dialog, once in response to the entry's 'activate' signal,
and again by activating the default button. Usually this is not a
problem, as the second invocation is simply ignored, however it breaks
the case where multiple consecutive questions are asked (e.g. username
and password in the 'Not listed' case).
Fix the problem by not handling the 'activate' signal at all.

https://bugzilla.gnome.org/show_bug.cgi?id=685511