Remove count_uploads_size feature flag

Identified with https://gitlab.com/gitlab-org/gitlab/-/jobs/916759016
Followup from https://gitlab.com/gitlab-org/gitlab/-/merge_requests/49998

config/feature_flags/development/count_uploads_size_in_storage_stats.yml

----
-name: count_uploads_size_in_storage_stats
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/46941
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/281950
-milestone: '13.6'
-type: development
-group: group::project management
-default_enabled: false