cmake: Remove old plugin file

[norihiro]

Description

This PR adds a post-install script to remove the old plugin whose name starts with "obs-".

How Has This Been Tested?

touch "$HOME/Library/Application Support/obs-studio/plugins/obs-simple-plugin-template.plugin"
installer -pkg package/simple-plugin-template.pkg -target CurrentUserHomeDirectory
test ! -e "$HOME/Library/Application Support/obs-studio/plugins/simple-plugin-template.plugin"

General checklist

• The commit is reviewed by yourself.
• The code is tested.
• Document is up to date or not necessary to be changed.
• I hereby declare that I have the legal right to contribute this work to the project under its specified open-source license, and I agree to the terms of the Developer Certificate of Origin.