[REF] Minor cleanup in CiviMail

[semseysandor]

Overview

• Remove unused local variables, block of code
• Remove deprecated CRM_Mailing_BAO_Mailing::getGroupNames (scheduled for removal: around 5.77)
• add missing throws tags
• use Civi::log()->error instead of CRM_Core_Error::debug_log_message

[civibot]

🤖 Thank you for contributing to CiviCRM! ❤️ We will need to test and review this PR. 👷

Introduction for new contributors...

• If this is your first PR, an admin will greenlight automated testing with the command ok to test or add to whitelist.
• A series of tests will automatically run. You can see the results at the bottom of this page (if there are any problems, it will include a link to see what went wrong).
• A demo site will be built where anyone can try out a version of CiviCRM that includes your changes.
• If this process needs to be repeated, an admin will issue the command test this please to rerun tests and build a new demo site.
• Before this PR can be merged, it needs to be reviewed. Please keep in mind that reviewers are volunteers, and their response time can vary from a few hours to a few weeks depending on their availability and their knowledge of this particular part of CiviCRM.
• A great way to speed up this process is to "trade reviews" with someone - find an open PR that you feel able to review, and leave a comment like "I'm reviewing this now, could you please review mine?" (include a link to yours). You don't have to wait for a response to get started (and you don't have to stop at one!) the more you review, the faster this process goes for everyone 😄
• To ensure that you are credited properly in the final release notes, please add yourself to contributor-key.yml
• For more information about contributing, see CONTRIBUTING.md.

Quick links for reviewers...

• ➡️ PR Demo site (can be rebuilt with the command test this please).
• 📖 Review standards
• 🗒️ Review template (brief or verbose)

➡️ Online demo of this PR 🔗

[totten]

Looking for callers, I see:

• getGroupNames is called by...
  • CRM_Utils_Token::getMailingTokenReplacement , which is called by:
    • CRM_Mailing_Tokens::evaluateToken(), and...
    • CRM_Utils_Token::replaceMailingTokens(), which is called by:
      • CRM_SMS_Form_Upload, and...
      • CRM_Mailing_Event_BAO_MailingEventResubscribe

(edited to use nested layout)

It might be that...

• The code-paths are dead for other/higher-level reason (in which case more can be cleaned), or....
• These specific callers lack test-coverage for the {mailing.group} token.

[totten]

This deprecation was suggested in f31f94b. @eileenmcnaughton, does this actually look safe to remove?