[kwinglutils] Drop hasEGLVersion function

Summary:
It's unused in KWin and absolutely not needed as the call to
eglInitialize, which every platform does, also gives the version for
free. So no need to do any caching in a global method.

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D3390