Issue-related email settings not properly validated

This bug dates back to when I split project_issue into a separate module and then later split the issues settings into their own subtab on the project edit page. :( The validation for the issue-related email addresses was left in project/project.inc's project_project_validate(), which was never firing, since those settings aren't in the main project node form anymore.

Attached patch does the following:
- Adds a proper validation handler for the issues subtab to validate these email addresses in project_issue.module.
- Rips out the dead code that was trying to validate email addresses in project_project_validate().
- Rips out other dead code in there doing silly things to validate/munge the checkboxes. That dates back to revision 1.5 of project.inc, circa 2003. ;) Begone, weird array_filter() code from long before FAPI...

Lest anyone think I'm being careless, I did take the time to try to figure out what this code was trying to do: it's a clumsy way of attempting to filter out bogus checkbox values. However, FAPI does that for us now. Moreover, array_filter(), if you don't give it a callback, will do the equivalent of what project_project_cleanup() was trying to accomplish. This dead validation code is the only place calling that function, so project_project_cleanup() itself is also killed by this patch.

Tested on a local HEAD site and validation is working again on those email addresses, and the checkboxes all behave properly.

Any objections before I commit?