Removed the "fastblur" path

Summary:
Since the new blur is more efficient "fastblur" or "simpleblur" is not needed anymore for fullscreen blur.
Even my old low-end laptop from 2009 (with Intel T3300) can easily do fullscreen blur now with the new method without any hitch at all.

Reviewers: graesslin, #kwin, #plasma, #vdg, fredrik

Reviewed By: #vdg, fredrik

Subscribers: avaragic, fredrik, ngraham, plasma-devel, kwin, #kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D10181