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.

django/core/management/base.py

@@ -386,7 +386,6 @@ class BaseCommand(object):
         if options.get('no_color'):
             self.style = no_style()
             self.stderr = OutputWrapper(options.get('stderr', sys.stderr))
-            os.environ[str("DJANGO_COLORS")] = str("nocolor")
         else:
             self.stderr = OutputWrapper(options.get('stderr', sys.stderr), self.style.ERROR)