Remove a few security_remote feature flag checks

1. Removes the ALLOWED_FAILURES constant from Security::Pipeline. These were specific to jobs defined in GitLab EE, and since we use production for security development, none of these jobs are allowed to fail anymore.
2. Removes the security_remote flag check from the security:validate and security:merge tasks. We are actively using production for security development for the foreseeable future, so it makes sense to always merge and validate on both Build and Security, while we continue to work on transitioning other projects off of Build.