Remove the 'emc' entry from config/exp.php and all conditional checks.
The Mastodon API compatibility stripping in StatusService and
AccountService is now unconditional.
- config/exp.php: remove emc entry
- StatusService: remove early-return when emc is false
- AccountService: remove early-return when emc is false
- Installer: stop writing EXP_EMC to .env
- .env.example, .env.docker.example: remove EXP_EMC
- Diagnostics: remove EXP_EMC row
