git: Unconditionally lower big file threshold

In 34399547 (git: Lower big file threshold to improve memory use and diff latency, 2023-08-08), we have introduced logic to lower the big file threshold of Git from the default 512MB to 50MB. BLobs larger than 50MB will thus be treated specially: they aren't diffed, they aren't deltified, and Git will use streaming interfaces instead of slurping any such blobs into memory directly. This should thus both reduce compute time and memory overhead in repositories that have such large blobs.

We have rolled out this flag into production a few days ago without any observed negative fallout. On the other hand, we also didn't observe any immediate positive consequences, either. This is expected though: the amount of requests that handle such files is going to be miniscule, so any effect caused by the lowered limit will be drowned out by all the other requests.

Let's remove the feature flag guarding this new setting.

Closes #5496 (closed).