graphics/py-opencv: drop python3 vestige after r422698

PYTHON_REL is defined in bsd.port.pre.mk, so this unlikely to ever have
worked unless defined via make.conf. Note, USES=python only supports
overriding PYTHON_VERSION apart from DEFAULT_VERSIONS=python*.

$ make -V PYTHON_VERSION PYTHON_REL=3500
python2.7

$ PYTHON_REL=3500 make -V PYTHON_REL
2712

$ make -V PYTHON_REL PYTHON_VERSION=python3.5
3502

PR: 204519 (for tracking)