Fixed #23107 -- Made runserver output respect --no-color.

This commit reverts 67d7da5fb9.

The previous fix changed the environment globally, which meant
that any call to `call_command(no_color=True)` prevented further
`call_command` with color.

This fix still relies on the environment because it's currently the only
way to reach WSGIRequestHandler, but it's now limited to the `runserver`
command. This seems an acceptable compromise considering `runserver` runs
indefinitely.

Thanks Tim Graham for the review.

tests/admin_scripts/tests.py

@@ -1397,7 +1397,6 @@ class CommandTypes(AdminScriptTestCase):
         out = StringIO()
 
         call_command('color_command', no_color=True, stdout=out)
-        self.assertEqual(os.environ.get('DJANGO_COLORS', ''), 'nocolor')
         self.assertEqual(out.getvalue(), 'BEGIN\n')
 
     def test_base_command(self):