edit-dialog : correct time row visibility

The revealer widget in edit dialog's event time interval row uses to hide the
time selector widgets when the event is checked as all-day and also reveal the
widgets when not checked as all-day.

Sometimes edit dialog may turn on the visibility of the time selector
widgets when the revealer's animation already starts, causing a flickering of the widgets.

We remove the revealer widget and leave edit dialog to toggle the
visibility of time selector widgets when needed.

https://bugzilla.gnome.org/show_bug.cgi?id=771572