edtlib: remove python 3.5 workaround

Remove a yaml monkeypatch. It is no longer needed since we support 3.6
or later on Zephyr v2.7 LTS and 3.8 or later on what will become v3.2.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>
(cherry picked from commit 7ef9c4b2)