frontend: Remove channel change signal handler from transform dialog

[derrod]

Description

Remove OBSChannelChanged from OBSBasicTransform.

Motivation and Context

This doesn't work. The primary channel is a transtiion, not a scene. This actually breaks the transform dialog if it's open while the transition is being changed.

How Has This Been Tested?

Compiled, still works.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• Code cleanup (non-breaking change which makes code smaller or more readable)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.