gdm: don't clear bullets while authenticating

gdm: don't clear bullets while authenticating

Users don't expect the bullets they just typed into an entry
field to disappear as soon as they hit enter.

Instead, they want the dialog to become insensitive during the
authentication process, so that it's clear that what they typed
in is being processed.

https://bugzilla.gnome.org/show_bug.cgi?id=657894