KisOpenGLModeProber: disable prober workaround for plasma only on 5.15+

This workaround causes Krita to crash on Windows with Qt 5.15.7.

The original patch (see commit ID below) reverts
8d6a2a5d wholesale. To preserve 5.1
compatibility, here I wrapped the removed bits in a version check,
so that they keep executing with 5.12.

CCBUG:408015
(cherry picked from commit ea5fde3d)