Revert "Fix #128257: Crash when using Set Vertex Colors with no brush"

This reverts commit 8d11882d.

The previous commit was misguided. As explained in #131893 and #131924
the desire for the various paint modes is to allow color operations
regardless of the tool that is currently selected.

Pull Request: https://projects.blender.org/blender/blender/pulls/131983