Remove deprecated image build log mechanism

[geropl]

In order to move image-builder to the workspace cluster, we need to ensure we use the same log streaming mechanism for image builds as for prebuilds.
This issues is about rolling this out to prod (feature flagged) to verify it actually does work, and then roll it out to all users.

Context: #9337

[geropl]

Re-opening because we have to cleanup the feature flag in ~a week.

[geropl]

Just checked the logs:

• there are no entries that the old code path has been taken the last 7 weeks (link)
• there are not entries that deprecating the new path triggered any errors during my manual tests (link)

Rolled this out to:

• staging & dev
• prod: Segment "Gitpod Internal" (waiting for rollout of this PR)

[geropl]

I noticed a "empty log" situation with an image-build for a prebuild the other day on the "Prebuild Details" view.
Status and "Skip Prebuild" were shown, but no logs appeared. Re-loading fixed it.

My suspicion is that either:

1. the getWorkspaceInfo thing failed. I thought we'd see an red error message in the log view, but maybe I missed sth?
2. the getImageBuildLogs did exit due to an escalated error
3. getImageBuildLogs registration got "removed" by a websocket disconnect

Update: This as a separate issue, and even a separate piece of code 🙈 So, not relevant here.

[geropl]

The changes have been rolled out. Only thing missing is a PR removing the feature flag and dead code.