Remove #mw_trackbacks CSS from shared.css.

This was historically used on [[MediaWiki:Trackbackbox]] (from where it was
moved to Article::addTrackbacks() in r47543, committed on 20 February 2009
by raymond), but the entire trackback feature was removed in r104051, committed on 23 November 2011.

The #mw_trackbacks definition was moved from various (skin-specific) CSS
files to here in r52404 (committed on 25 June 2009) by ialex.

Change-Id: I7809de83ebb537d4739ef89265c32935aa036e1d