[pyroot] Do not silently set `EXTRA_CLING_ARGS` to `-O2`

[eguiraud]

import ROOT causes the setting of the environment variable EXTRA_CLING_ARGS to -O2 -- but too late (after interpreter initialization), so that the setting actually has no effect: cling will still use its default of -O1, but checking EXTRA_CLING_ARGS will give users the wrong impression.

Since there is no reason to set the environment variable in the first place, it has no effect anyway, and it can be confusing for users trying to figure out what options cling is running with, I propose to remove the behavior from pyroot.