Clean joystick code in user_input.cpp

[ssejrog]

The code below is repeated in arcade_flipped, arcade_standard, and tank.

  // Threshold if joysticks don't come back to perfect 0
  if (abs(l_stick) > JOYSTICK_THRESHOLD || abs(r_stick) > JOYSTICK_THRESHOLD) {
    set_tank(r_stick + l_stick, r_stick - l_stick);
    reset_drive_sensor();
  }
  // When joys are released, run active brake (P) on drive
  else {
    set_tank((0 - left_sensor()) * active_brake_kp, (0 - right_sensor()) * active_brake_kp);
  }

Also, there's no reason to reset the drive sensors in opcontrol unless active brake is being used. if (active_brake_kp != 0) should be in front of reset_drive_sensor().