Remove useless os.path.exists check for brctl executable

The networking module previously checked that its hard-coded path
for the brctl command existed before collecting output. This was
missed in the conversion to using policy defined PATH search for
external commands in commit e0d132e.

Similar problems were fixed in commits 8b10cb0, 374da99, 8883155.

Signed-off-by: Bryn M. Reeves <bmr@redhat.com>