Fix the po-box spec function

[Powersource]

From https://github.com/ssbc/ssb-tribes/pull/96/files#r1377391331

in the listener file we tried to simplify the different listeners by simply filtering using the spec functions like isPOBox etc. instead of also filtering manually by message type. It worked for add-member and exclude-member but for the po-boxes it seemed to let weird messages through, see linked thread above.