master: Obsolete dependency for Swiss maps

Go 1.24 and 1.25 have Swiss maps enabled by default, with option to disable them via `GOEXPERIMENT=noswissmap` environment variable.
Upcoming Go 1.26 and later will have Swiss maps always enabled.

Since pebble (on master branch) already requires at least Go 1.25:
https://github.com/cockroachdb/pebble/blob/4bce934de56ebe5741720d9b2424065317e42bf4/go.mod#L60
...the dependancy on [`github.com/cockroachdb/swiss`](https://github.com/cockroachdb/swiss) Go module for Swiss maps should no longer be needed.

This would lower the maintenance with every future Go release.

Jira issue: PEBBLE-1312

Start with go.mod at the referenced revision and inspect the github.com/cockroachdb/swiss requirement and related dependency entries. Confirm the module is no longer needed with the repository's minimum Go 1.25 version, then run the project's Go tests or build checks. Done means the obsolete dependency is removed without breaking dependency resolution or tests.

Written by the indexing model from the issue text.