Scrape away last of the GLEW

Summary:
This removes the GLEW requirements in kis_mirror_axis.cpp. There were a
few references to GLEW from previous patches that needed to be tidied
up.

Test Plan: Mirroring canvas with "M" shortcut continues to work correctly.

Reviewers: dkazakov, rempt

Maniphest Tasks: T360

Differential Revision: https://phabricator.kde.org/D239