Complete de"qobject"ing KisOpenGLImageTextures

Summary:
KisOpenGLImageTextures has no signals, so the only purpose of
disconnectCurrentCanvas() is a noop.
And it's two slots are nowhere used as slots, but only called directly.

Kept the names of the former slots for consistency with KisPrescaledProjection
as used in QPainter canvas

Reviewers: rempt, dkazakov

Reviewed By: rempt, dkazakov

Differential Revision: https://phabricator.kde.org/D661