Remove unused low-level netcode stuff

The low-level parts in SRB2's netcode had some unused stuff, some of which seems to have been used in the past but no longer used while others seem to be incomplete. This patch removes some of this to clean up some of the mess in the netcode, and also prepares for removing the doomcom global variable later and passing it via arguments instead.

added Netcode Refactoring small labels

merged

mentioned in commit ff1eae14