ssh: Fix semantic merge conflict with testcfg changes

The testcfg package has recently been converted to use t.Cleanup() instead of the testhelper.Deferrer() in commit 0fc3e28a (Merge branch 'smh-remove-deferrer' into 'master', 2021-03-15). This has caused a semantic merge conflict with 36673907 (Merge branch 'ps-rm-config-ssh' into 'master', 2021-03-16).

Fix the semantic merge conflict by dropping the removed cleanup return value.