SITL: remove dead stores from rangefinder_range

[peterbarker]

roll and pitch are unused if !SITL_RANGEFINDER_AS_OBJECT_SENSOR

... so move roll/pitch declaration and manipulation into the #if block where they are used

../../libraries/SITL/SIM_Aircraft.cpp:546:13: warning: Value stored to 'roll' is never read [deadcode.DeadStores]
            roll = 0;
            ^      ~
../../libraries/SITL/SIM_Aircraft.cpp:551:13: warning: Value stored to 'roll' is never read [deadcode.DeadStores]
            roll = 0;
            ^      ~
../../libraries/SITL/SIM_Aircraft.cpp:557:13: warning: Value stored to 'pitch' is never read [deadcode.DeadStores]
            pitch = 0;
            ^       ~
../../libraries/SITL/SIM_Aircraft.cpp:562:13: warning: Value stored to 'pitch' is never read [deadcode.DeadStores]
            pitch = 0;