Remove unneeded wait for notification message

There is a confict between waits
There is a <code>self.wait_for_element_not_displayed(*self._notification_locator)</code> here:
https://github.com/mozilla/gaia-ui-tests/blob/v1-train/gaiatest/apps/marketplace/regions/settings.py#L55
and then a <code>marketplace.wait_for_notification_message_displayed()</code> here: https://github.com/mozilla/gaia-ui-tests/blob/master/gaiatest/tests/marketplace/test_marketplace_change_region_anonymous.py#L33
So after tapping on "Save changes" there is no notification message to wait for, making both 
<code>test_marketplace_change_region_anonymous</code> and <code>test_marketplace_change_region_login</code> to fail