MWReferenceGroupInputWidget: Remove dummy call to onInputChange()

This was needed to make the ComboBoxWidget initialize
its selection, but Idcbf38c53 makes that happen automatically.

Depends on Idcbf38c53 in oojs-ui.

Bug: 72783
Change-Id: I8d3a014f862ece7f4f201af205a8a7ca6c365eaa
Merge "MWReferenceGroupInputWidget: Remove dummy call to onInputChange()"