Fixed defender's objective tracking flag carrier ID

Removed DEFENDERS_OBJECTIVE_ONFLAG and DEFENDERS_OBJECTIVE_ONCARRIER and all associated updates/calls from base_id.lua
	- defenders objective should point to cap always to avoid tracing the flag carrier 
fortressforever/fortressforever#58
This line is unnecessary and is very likely to become stale/inaccurate (see 99% of the FF .cpp headers that are wildly inaccurate). Version control keeps track of this, so there's no need to put it in the file.