UI: Remove legacy migrations

[derrod]

Description

Removes some legacy cruft.

Motivation and Context

The FFmpeg output and reconnect setting migrations are over 9 years old.

The NVENC preset migrations are also over 2 years old, and are currently also handled in the encoder still.

How Has This Been Tested?

OBS builds and runs.

Types of changes

• Code cleanup (non-breaking change which makes code smaller or more readable)

Checklist:

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