Remove unused module requirement

Summary:
It was added for when FolderView did some low level GL stuff to fetch an
image for thumbnails, but now this uses higher level API.

Find -name CMakeLists.txt | xargs grep OpenGL returns nothing else

Test Plan:
Grepped for usages
Compiles fine

Reviewers: #plasma, mart

Reviewed By: #plasma, mart

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D9150