Remove unused files and assets

[yutotakano]

Removes:

• GitPod setup (last used several years ago even in upstream, never used in our fork)
• GitLab CI file (we have moved to GH Actions)
• SIP Deployment description file (upstream uses this for deployment, we use k8s manually)
• Old favicon (while our meta tags used BI favicons for a while, the old file was still there. We now remove that too)