Replace vendored uthash with prefix/system install

[tytan652]

Description

Requires:

• https://github.com/obsproject/obs-deps/pull/203
• https://github.com/obsproject/obs-studio/pull/10150

Replace vendored uthash with prefix/system install.

Motivation and Context

Less vendored dependencies

How Has This Been Tested?

Builds on ArchLinux with uthash package and Flatpak builds. The CI itself.

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.

[PatTheMav]

@tytan652 you might wanna rebase this as I think we might be able to merge this soon-ish.