aja: Remove holding source settings

[norihiro]

Description

This PR removes AJASource::CaptureThread to hold the settings of the source.

Waiting

• docs: Clarify obs_source_update can accept NULL for settings argument #11189

Motivation and Context

The API obs_source_update accept nullptr for the argument settings and it will just call the update callback without changing the settings.

The usages of obs_source_update seems to trigger the update callback in the graphics thread.
For that purpose, it is sufficient to pass nullptr as the 2nd argument.

How Has This Been Tested?

I haven't test it as I couldn't reached the branch.

Types of changes

• 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.