Mentors are no longer required

As of the September Eclipse Foundation Development Process 1.11 release, mentors are no longer required.

Any mentor information that is interesting/useful is in the Foundation database, so there's no reason to keep the field around.

The getMissingCreationRequiredValues function in Project_Proposal.class.inc calls a method, getMentors and checks that there is at least one value in the results. If this test is removed, then we can at least leave the field blank while you work on removing the rest.

When all references to mentors are removed from the project and project_proposal types, the "mentor" instance of the project_role_type content type can be deleted.