MWCategoryWidget: Use TextInputWidget#event-enter instead of keypress

The TextInputWidget class inherited by MWCategoryInputWidget,
already has a keypress listener that emits an "enter" event on
the OO.ui.Element object for us to listen to. No need to duplicate
that logic again.

Change-Id: Ia47bc8976f22dfe7e3a6fb6068dc4b6e03a3357b
Merge "MWCategoryWidget: Use TextInputWidget#event-enter instead of keypress"