cmake: drop ERROR_QUIET in python.cmake

ERROR_QUIET should be used only when the command is actually expected to
fail, otherwise discarding stderr can be extremely time-consuming. See
recent example of time lost in commit 40c2e08e ("xcc/cmake: don't
discard stderr; don't (ever!) use ERROR_QUIET")

If some Python interpreter cannot even evaluate `sys.version_info[:2]`,
then we definitely don't want to hide what its stderr says about that.

Fixes commit 94de3e9f ("cmake: FindPython3: Adjust python3 program
search")

Signed-off-by: Marc Herbert <marc.herbert@intel.com>