RichtextEditor: don't rehighlight() directly, once is enough.

Summary:
With the sonnet fix https://phabricator.kde.org/D14447,
setLanguage takes care of scheduling a call to rehighlight (via a timer
which is already started in the constructor anyway),
so that we don't have to call it directly here, leading to two calls.

The hackish processEvents() has to be removed too, since otherwise it
would process the ctor timer and rehighlight before the call to
setLanguage even happens. I think the 500ms timer below that
processEvents() call means that we don't need it anyway.

Test Plan:
Replying to a mail in kmail, and checking how many times
slotRehighlight is called. With and without autodetection of language.

Reviewers: sandsmark, mlaurent

Reviewed By: mlaurent

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D14448