OptionGroup - Deprecate isDomainOptionGroup function

Now that the last 2 domain-specific option groups have been dealt with:

- https://github.com/civicrm/civicrm-core/pull/31909
- https://github.com/civicrm/civicrm-core/pull/31924

... we can stop using this godawful is_domain column for good.
The OptionValue table is complex enough without it!