housekeeping: Remove accessor trigger

Relates to https://gitlab.com/gitlab-com/feature-change-locks/-/work_items/126

The housekeeping middleware was modified in 13cbff25 (housekeeping: Add repo stats collection to housekeeping, 2025-08-29) to optionally trigger on accessor RPCs. The primary reasoning at the time was to measure the number of files and directories (dentries) in a given repository, and to proactive perform housekeeping on them in order to reduce the number of dentries.

This was especially important during the transactions/WAL work, since the time taken to create a repository snapshot is directly tied to the dentry count.

Since we're no longer pursuing this work, we no longer care about the dentry count as it relates to housekeeping. We have also seen recently [1] that collecting the dentry count can be very expensive when the count is very high.

Let's remove the triggers for housekeeping that act on accessor RPCs. Repositories that are active (i.e. receive mutator requests) will continue to be housekept as usual.

[1] https://gitlab.com/gitlab-com/gl-infra/production/-/work_items/22971