wikibits: Replace checkboxShiftClick globals with dummies

They have been deprecated in favour of jquery.checkboxShiftClick for
many releases now (since MediaWiki 1.17) and aren't used anywhere in
MediaWiki core (or extensions that I know of).

Though removal is justified, given the slow pace at which the tour[1]
is progressing and the amount of garbage I still find on a daily basis,
I'm replacing them with dummies instead and marking the dummies as
deprecated for easier tracking.

[1] https://meta.wikimedia.org/wiki/User:Krinkle/Le_Tour_de_Wik%C3%AD/2011_Resource_Walker

Change-Id: I72aac6111b9be974f160fecad8312c49c9460c5b