doc: remove line about devserver pytest marker

The usage of this marker is outdated. Some tests now
requires the tests services in container not dev servers.
This marker has not been used in recently written tests

Author: candleindark

CLAUDE.md

@@ -130,7 +130,6 @@ The `datalad_registry_client` package provides a DataLad extension for interacti
 
 - Tests use `env.test` configuration with separate containers
 - Database is ephemeral for test runs
-- Use `@pytest.mark.devserver` for tests requiring Flask dev server
 - Use `@pytest.mark.slow` for long-running tests
 - Test fixtures available in `conftest.py` files