Blender Git Loki

Sequencer: remove effect mask input

Initial idea of this input was re-designed in a bit more flexible
way using modifiers.
Also since Color Balance (which was the only thing using effect
mask input) was moved to the modifiers, this input field became
rudiment.

It's pretty tricky to write versioning code to prevent possible
data in cases this field was used, but hope it wouldn't be difficult
to switch to modifiers masks.