Remove the unused 'jquery.farbtastic' module
This was added in (r99923;
4d8c7e96ed1f) while reviewing the
'gadget-preferences' branch in SVN of the Gadgets extension during
development of MediaWiki 1.18, intended for use in a color picker
on Special:Gadgets as part of an experimental feature that was
never completed (r94051).
The module has no usage anywhere in Wikimedia Git, nor in any
third-party hosted repos indexed by MediaWiki Codesearch, nor
anywhere on-wiki at WMF in site scripts (per mwgrep).
Bug: T193826
Bug: T192623
Change-Id: I8ed6c09dc7efd750ad4688b895b2e3f808a0e52b