Checkmk Werk 14772: Fix crash when deleting custom MIB in EC

ID: 14772 Title: Fix crash when deleting custom MIB in EC Component: Setup Level: 1 Class: Bug fix Version: 2.2.0i1 Attempting to delete a MIB under "SNMP MIBs for trap translation" of the event console would fail and crash with the error <tt> TypeError (unsupported operand type(s) for +: 'PosixPath' and 'str') </tt> This has been fixed.