Remove dynamic_image_resizing feature flag

Description

I'm reviewing some of the grouptenant scale feature flags.

In this case, dynamic_image_resizing feature flag is enabled for everyone on GitLab.com. I asked on Slack (message) and @ayufan confirmed. It's also available for self-manage because we have default_enabled: true on the YAML file.

We can remove the feature flag now.

Related to #266986 (closed) and #408376 (closed).