patron cleaning - do not delete staff borrowers

patron cleaning - do not delete staff borrowers

Adjusted C4::Members::GetBorrowersWhoHaveNotBorrowedSince so
that staff member records are not selected for deletion.

This functionality change is motivated by the consideration
that a staff member who is authorized to operate Koha is
not necessarily a prolific user of the library's services.
If a batch purge job for inactive staff records is really need,
it should be a separate tool.

Signed-off-by: Joshua Ferraro <jmf@liblime.com>