Revert code completion changes
This re-applies the following reverts in bulk:
3c22033 - Revert "Add assistants to the "argument hints" code completion" (3 months ago) <Milian Wolff>
eebb47f - Revert "Remove assistant popup" (3 months ago) <Milian Wolff>
9cbe01a - Revert "Move assistant completion to its own KTextEditor::CompletionModel" (3 months ago) <Milian Wolff>
As discussed on the mailing list:
https://mail.kde.org/pipermail/kdevelop-devel/2016-January/052737.html
Differential Revision: https://phabricator.kde.org/D857