kvclient: evaluate removing transaction pipelining

**Is your feature request related to a problem? Please describe.**

Now that we've got buffered writes, it mostly subsumes transaction pipelining -- in the common case, we're no longer writing intents as a transaction proceeds; we're also not writing replicated locks as a transaction proceeds, as those are intercepted by the write buffer and transformed into unreplicated locks (replicated locks are still flushed, if need be). This means that in most cases, we're not pipelining. This opens up the opportunity to get rid of the pipeliner interceptor.

We should evaluate whether there are workloads that benefit from being run with transaction pipelining instead of buffered writes; if there aren't any regressions, we should be able to get rid of the pipeliner entirely.

cc @stevendanna @miraradeva

Jira issue: CRDB-52340