Remove usage of `maunium.net/go/mautrix`

We should probably take a look at removing this dependency, as we solely use it in tests.

_Originally posted by @S7evinK in https://github.com/element-hq/dendrite/issues/3504#issuecomment-2616686423_

Start by locating the go.mod entry and all test references to maunium.net/go/mautrix. Confirm that production code does not use it, remove the unused dependency and any test-only references that require it, then run the relevant Go tests and dependency checks to verify the repository remains clean.

Written by the indexing model from the issue text.