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 UpdateObjectiveIcon call being in the if statement doesn't handle all cases correctly. For example, if DEFENDERS_OBJECTIVE_ONFLAG is false, but DEFENDERS_OBJECTIVE_ONCARRIER is true, then the DEFENDERS_OBJECTIVE_ENTITY would remain on the flag after they drop the flag.

I think it might be better to pull this logic out into a function or two to keep it consistent and avoid repetition. Something like:
```lua
-- alternatively, get the flag entity and check if it's carried instead of passing carried as a parameter
function GetDefendersFlagObjective(carried)
	if (not carried and DEFENDERS_OBJECTIVE_ONFLAG) or (carried and DEFENDERS_OBJECTIVE_ONCARRIER) then
		return GetEntityByName("cp"..phase.."_flag")
	else
		return GetDefendersCapObjectiveIcon()
	end
end

-- potentially add a DEFENDERS_OBJECTIVE_ONCAP variable and return nil here if it's set to false
function GetDefendersCapObjective()
	return GetEntityByName("cp"..phase.."_cap")
end
```

Then in the various flag drop functions:
```lua
DEFENDERS_OBJECTIVE_ENTITY = GetDefendersFlagObjective(false)
UpdateTeamObjectiveIcon( GetTeam(defenders), DEFENDERS_OBJECTIVE_ENTITY )
```
and in the flag pickup:
```lua
DEFENDERS_OBJECTIVE_ENTITY = GetDefendersFlagObjective(true)
UpdateTeamObjectiveIcon( GetTeam(defenders), DEFENDERS_OBJECTIVE_ENTITY )
```
and in the initial setup/oncap/etc functions:
```lua
DEFENDERS_OBJECTIVE_ENTITY = GetDefendersCapObjective()
UpdateTeamObjectiveIcon( GetTeam(defenders), DEFENDERS_OBJECTIVE_ENTITY )
```