[kwinglutils] Drop functions related to (nearest) power of two

Summary:
Not used anywhere in KWin and annotated that they should be removed. As
we have an ABI break in 5.9 we can remove them now.

Reviewers: KWin, Plasma

Subscribers: plasma-devel, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D3398