drop unused jquery.placeholder

Another cleanup. I will test placeholders with IE8 once the new VM is downloaded. Otherwise I couldn't find any uses of this jquery plugin in core, core apps or some other apps (contacts, calendar, news, ...)

We use placholders.js (I know there is also placeholder.js, but that is the imageplaceholder - the letter with the colored background).

cc @DeepDiver1975 @PVince81 @LukasReschke @jancborchardt