git: Drop the boundary bitmap traversal experiment

Quite a while ago in 678fcadf (git: Speed up git-upload-pack(1) via new bitmap boundary traversal, 2023-09-27) we have introduced a feature flag that makes git-upload-pack(1) use a new boundary-based bitmap traversal algorithm. This new Git feature was supposed to speed up certain cases where the old algortihm performed badly.

We couldn't observe any noticeable improvement to performance though. On the contrary, we have received some reports where git-upload-pack(1) now pushed significantly too many objects. While it is known that the new traversal may cause us to enumerate too many objects in some cases, the extent was that we basically ended sending almost the complete object graph for a fetch that should've led to only a few objects being sent.

Let's declare this experiment as failed for now. We didn't observe any positive effects but did observe a small handful of negative effects. Remove the feature flag and the code it was guarding.

Closes #5537 (closed).

Closes #5620 (closed).