Delete the redirect from /admin/application_settings to /admin/application_settings/general

Summary

Delete the redirect from /admin/application_settings to /admin/application_settings/general in GitLab 13.0.

The !22252 (merged) made the former path deprecated, but deferred its deletion until 13.0 release and introduced a temporary redirect. This issue is a reminder to delete this redirect.

Relevant discussion: !22252 (comment 270927994)

Improvements

Less redirects, less complexity. And this one was designed to be temporary.

Risks

None.

Involved components

config/routes/admin.rb