Remove 'Meta' category from abnf.js
Remove 'Meta' category from abnf.js

Categories are for grouping several languages in the demo page. 'Meta'
was the category with only ABNF in it.

MakeNowJust/highlight.js,palmin/highlight.js,aurusov/highlight.js,aurusov/highlight.js,StanislawSwierc/highlight.js,aurusov/highlight.js,carlokok/highlight.js,carlokok/highlight.js,highlightjs/highlight.js,highlightjs/highlight.js,isagalaev/highlight.js,Sannis/highlight.js,carlokok/highlight.js,lead-auth/highlight.js,carlokok/highlight.js,StanislawSwierc/highlight.js,MakeNowJust/highlight.js,Sannis/highlight.js,MakeNowJust/highlight.js,palmin/highlight.js,highlightjs/highlight.js,highlightjs/highlight.js,palmin/highlight.js,isagalaev/highlight.js,Sannis/highlight.js