Delete src/ui/widgets/ve.ui.ContextWidget.js

Unused. Follow-up to b141a7d23fc47938030cb6a742db51229770025e,
which copied a file instead of renaming.

Change-Id: I3ed8617b03b953b5c0e6d5374428329289272aec
Tweak some ContextSelectWidget code after item→option update

Follow-up to 029fc7e22cfc551f5bde344d78bf734178a20387. Seems to have
slipped through the cracks, caused no visible issues.

Change-Id: I3d28feec3b3202dfdc771ea771d138e5e8bb00d7