Firefox extension: remove unused preference cleanup from `bootstrap.js`
Firefox extension: remove unused preference cleanup from `bootstrap.js`

The comment for the removal has been added three years ago, so we can
safely remove this now.

timvandermeij/pdf.js,Snuffleupagus/pdf.js,mozilla/pdf.js,mukulmishra18/pdf.js,nawawi/pdf.js,xavier114fch/pdf.js,mainegreen/pdf.js,mainegreen/pdf.js,Shoobx/pdf.js,nawawi/pdf.js,tinderboxhq/pdf.js,mozilla/pdf.js,caremerge/pdf.js,tinderboxhq/pdf.js,ydfzgyj/pdf.js,Snuffleupagus/pdf.js,macroplant/pdf.js,adlerweb/pdf.js-Touch,showpad/mozilla-pdf.js,adlerweb/pdf.js-Touch,Rob--W/pdf.js,mozilla/pdf.js,timvandermeij/pdf.js,showpad/mozilla-pdf.js,websirnik/pdf.js,Rob--W/pdf.js,nawawi/pdf.js,1and1/pdf.js,websirnik/pdf.js,caremerge/pdf.js,xavier114fch/pdf.js,Shoobx/pdf.js,ydfzgyj/pdf.js,skalnik/pdf.js,macroplant/pdf.js,Snuffleupagus/pdf.js,1and1/pdf.js,Shoobx/pdf.js,macroplant/pdf.js,mukulmishra18/pdf.js,showpad/mozilla-pdf.js,skalnik/pdf.js,timvandermeij/pdf.js