UI: Remove duplicate missing files code

[cg2121]

Description

The same code was in different places, so call single function to
cleanup code.

Motivation and Context

Code cleanup.

How Has This Been Tested?

Tested with missing files to make sure dialog still worked properly.

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.

[norihiro]

There was a bug that the missing file dialog cannot catch missing files on transitions. However, this PR fixed the issue.

The step to reproduce the bug is as below.

1. Add a stinger transition. Set a video file.
2. Exit OBS
3. Remove the video file.
4. Start OBS
   • Without this change, Missing Files dialog does not appear.
   • With this change, Missing Files dialog report a file is missing from the stinger transition.
5. With this change, update the file path on the dialog.
6. Run the stinger transition and confirmed the updated video file is played.

I've made another PR #6753 but the PR #6753 is unnecessary if this PR is merged.

[norihiro]

I'm afraid this change is dangerous in terms of thread safety. Prior to this change, the sources have not been revealed to the graphics thread when checking the missing files. Now, the missing files are checked after the graphics thread started to access the sources. The graphics thread also touch the data when defer_update_count is set.

Especially, some sources such as clut_filter calls obs_source_update during the create callback, which triggers first tick in the graphics thread to call update callback and modify pointers for example char *file in clut_filter. Assuming #5401 will be merged, UI thread will read the same pointer so that this is a race condition.

I'd like to suggest reverting this change and also do not call obs_source_update from the create callback but directly call each callback function for update.