plugins/obs-ffmpeg: remove restart guard for RIST

[RossWang]

Description

Remove restart guard. Media Source running RIST can shutdown successfully when it becomes inactive.

Motivation and Context

Media Source with RIST protocol consuming 1 CPU core at 100% when not active. Updated to the latest library version solved the CPU issue but it is still running in the background which is not ideal. I think the original intention is to prevent crash. After years of development, librist is much stable now. The guard can be removed.

How Has This Been Tested?

• Windows 10
• MSVC build
  (librist in obs-deps need to be updated accordingly)

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

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