Removes sse_image_uploads, feature flag usage removed

See https://gitlab.com/gitlab-org/gitlab/-/issues/282534

config/feature_flags/development/sse_image_uploads.yml

----
-name: sse_image_uploads
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/35126
-rollout_issue_url: 
-milestone: '13.2'
-type: development
-group: group::static site editor
-default_enabled: false