Account for categories being removed twice in MWCategoryWidget

Now that we use staging in the meta dialog, the following
sequence is possible:
* Add Category:Foo in UI (adds to DM and adds a widget)
* Remove Category:Foo in UI (removes from DM, removes widget)
* Click Cancel to leave meta dialog
** popStaging()
*** Undo removal of Foo (adds to DM, does not add a widget)
*** Undo addition of Foo (removes from DM, tries to remove nonexistent widget)

Add a check so trying to remove an already-removed widget
doesn't cause a JS error.

Bug: 71471
Change-Id: I34690364ce302b858e2a4429dbb97b57d39aae5f
(cherry picked from commit ff587f4f61612a49e2f77904ce1a7744f19c8d39)