Issue #3112419: Remove unused variables

Summary

Removes unused variables reported in issue #3112419:

1. Unused $output initialisation in redirect_help().
2. Unused $output initialisation in redirect_domain_help().
3. Unused $storage_definition assignment in redirect_update_8101().
4. Unused $message initialisation in redirect_update_8103().

Testing

Ran:

• php -l redirect.install
• php -l redirect.module
• php -l modules/redirect_domain/redirect_domain.module
• git diff --check

All checks passed.

Notes

This ports the existing patch from the issue queue into the issue fork source branch so it can be reviewed as an MR.

Closes #3112419