cmake: Remove setting code sign identity for Xcode builds

[PatTheMav]

Description

This is a companion commit to fix the team-based code signing in Xcode enabled by https://github.com/obsproject/obs-studio/pull/6934

Motivation and Context

Using a team identifier with automatic code signing allows development teams to use stable code signatures without requiring each member to have their own Apple Developer membership.

How Has This Been Tested?

Tested on macOS 12.5.

Types of changes

• Tweak (non-breaking change to improve existing functionality)

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.