MuonCablingTools+RPC_CondCabling: remove MsgStream from BaseObject

This finalizes the removal of the MsgStream instances in the muon cabling BaseObject class. Previously, derived classes were able to use the DISP macros for messaging. All those uses have been replaced by other means of messaging (see !45581 (merged), !45582 (merged), !45597 (merged)). As we have O(1M) instances of these classes, this significantly reduces the size and time needed to initialize them.

Closes ATLASRECTS-6313.

Also remove ancient gcc 2.x compatibility code and the obsolete __osstream typedef.

cc @jojungge