Fix kmail autocompletion from akonadi.

My commit 02f5f021 made autocompletion from nepomuk work better, but broke
completion from akonadi. I kept the "keywords" based code, but now it's only
used for the special case of nickname-based search (because the nickname shouldn't
appear in the completion item). For everything else it really doesn't make sense
to have a search engine (akonadi/nepomuk) on top of a search engine
(the one inside KCompletion).

This time I verified that:
* nepomuk search still works
* contacts from akonadi work again
* contact groups from akonadi work (after previous commit)
* nickname-search in akonadi still doesn't work, but it didn't before. More work
needed for that one. This is the only reason to keep KMailCompletion around btw,
everything else would work without it.

BUG: 259949
FIXED-IN: 4.10.3