site_mission cruft in system.test and possibly in system.token.inc?

#428800: Convert mission statements to a region with blocks took out the site mission statement ("Your site's mission or focus statement (often prominently displayed on the front page).") in favor of letting users do this manually with a custom block (if i understand it correctly).

The site_mission variable still shows up in a few spots where it seems it shouldn't:

drupal/modules/system/system.tokens.inc:168: $mission = variable_get('site_mission', '');
drupal/modules/system/system.test:1507: variable_set('site_mission', 'Mission');
drupal/modules/system/system.test:1513: $tests['[site:mission]'] = filter_xss(variable_get('site_mission', ''));
drupal/modules/system/system.test:1530: $tests['[site:mission]'] = variable_get('site_mission', '');

(Note: It is also in block.install but here it is used properly to convert legacy mission statements to the new block method, and then deletes it for good.)

For system.token.inc, I think the rss site_description should be made a token, since it is the half-successor to the site_mission variable.

Confirmation, and we can roll a patch or two.

_{benjamin, agaric}