Remove Builds tab from Merge Requests and Commits

Description

The Builds tab is a legacy holdout from before Pipelines. It's continued existence is making design and development more complicated. We were planning on waiting until 9.0 to remove it, but it's been long enough and people are fully embracing Pipelines so let's remove the Builds tab from Merge Requests and Commits (and anywhere else it might appear, other than the new usage in #21953 (closed)).

Proposal

• Remove the Builds tab from Merge Requests and Commits (and anywhere else it might appear, other than the new usage in #21953 (closed)).

Links / references

• Somewhat blocked on #19703 (closed), although in a pinch we could ship it anyway. #19703 (closed) somewhat compensates for removing the build list.