Remove deprecated stuff

Removed things deprecated in Adora Belle, Angua (and even Anteater). See https://www.dokuwiki.org/devel:releases Although according to plan this should be part of the next release (Binky), I guess it would also be okay to merge after the release.

I haven't removed the following:
* "double-decoding in XMLRPC putAttachment" as I don't know that part of the code and wasn't sure what exactly was deprecated
* "disabled plugin files" as I don't understand that code
* "DOKU_TPL and DOKU_TPLINC constants" because they are still used by the vast majority of all templates (about 85%), so shouldn't be removed

Would someone like to tackle the first two of those please?