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
Actually, it'd be better to just do something like this (as per the comments in my last comment's code):

```lua
function UpdateDefendersObjective()
    local flag = GetEntityByName("cp"..phase.."_flag")
    -- not sure if IsCarried exists, just going by memory at the moment
    local carried = flag:IsCarried()
    if (not carried and DEFENDERS_OBJECTIVE_ONFLAG) or (carried and DEFENDERS_OBJECTIVE_ONCARRIER) then
        DEFENDERS_OBJECTIVE_ENTITY = flag
    elseif DEFENDERS_OBJECTIVE_ONCAP then
        DEFENDERS_OBJECTIVE_ENTITY = GetEntityByName("cp"..phase.."_cap")
    else
        DEFENDERS_OBJECTIVE_ENTITY = nil
    end
    UpdateTeamObjectiveIcon( GetTeam(defenders), DEFENDERS_OBJECTIVE_ENTITY )
end
```
and then simply call whenever the objective icon would need to change.