pybind/mgr: remove coverage env from default env list

Keeping coverage in this list just causes make check to run the same set
of tests twice, with one time being slightly slower - and no one is
looking at the coverage report in make check.
Drop the env from the default list so it will only be run upon request
rather than with every default tox invocation.

Signed-off-by: John Mulligan <jmulligan@redhat.com>