Remove contrib/jsonb_plpython's tests for infinity and NaN conversions.

Remove contrib/jsonb_plpython's tests for infinity and NaN conversions.

These tests don't work reliably with pre-2.6 Python versions, since
Python code like float('inf') was not guaranteed to work before that,
even granting an IEEE-compliant platform.

Since there's no explicit handling of these cases in jsonb_plpython,
we're not adding any real code coverage by testing them, and thus
it doesn't seem to make sense to go to any great lengths to work
around the test instability.

Discussion: https://postgr.es/m/E1f1AMU-00031c-9N@gemulon.postgresql.org