URL aliases not cleaned up when projects are deleted

See #431326: URL aliases not removed for some legacy project nodes that were deleted and #430528: Flag Form module short_path URL doesn't resolve for background.

Poking around in the {url_alias} table on d.o, I notice there are a fair number of URL aliases from this module that belong to project nodes that no longer exist:

mysql> SELECT count(*) FROM url_alias u LEFT JOIN project_projects pp ON u.src = CONCAT('node/', pp.nid) WHERE pp.nid IS NULL AND u.dst RLIKE 'cvs-instructions';

+----------+
| count(*) |
+----------+
|      804 | 
+----------+
1 row in set (3.24 sec)

path.module isn't going to automatically delete these for you when the node is deleted, since the alias isn't directly to node/N. So, we need a hook_nodeapi() in here to clean out the cvs-instructions alias when a project node is deleted, and we should probably have a DB update to clean out stale aliases while we're at it.