message: gimbal integration tests: remove unnecessary std::bind
diff: #include "plugins/gimbal/gimbal.h"
  #include "plugins/offboard/offboard.h"
  -using namespace std::placeholders; // for `_1`
  using namespace dronecore;
  @@ -38,7 +37,7 @@ TEST_F(SitlTest, GimbalMove)
  telemetry->set_rate_camera_attitude(10.0);
  telemetry->camera_attitude_euler_angle_async(
  - std::bind(&receive_gimbal_attitude_euler_angles, _1));
  + &receive_gimbal_attitude_euler_angles);
  for (int i = 0; i < 500; i += 1) {
  @@ -73,7 +72,7 @@ TEST_F(SitlTest, GimbalTakeoffAndMove)
  telemetry->set_rate_camera_attitude(10.0);
  telemetry->camera_attitude_euler_angle_async(
  - std::bind(&receive_gimbal_attitude_euler_angles, _1));
  + &receive_gimbal_attitude_euler_angles);
  for (int i = 0; i < 500; i += 1) {
  @@ -121,7 +120,7 @@ TEST_F(SitlTest, GimbalROIOffboard)
  telemetry->set_rate_camera_attitude(10.0);
  telemetry->camera_attitude_euler_angle_async(
  - std::bind(&receive_gimbal_attitude_euler_angles, _1));
  + &receive_gimbal_attitude_euler_angles);
  // Send it once before starting offboard, otherwise it will be rejected.
  offboard->set_velocity_ned({0.0f, 0.0f, 0.0f, 0.0f});
  @@ -162,15 +161,14 @@ void send_new_gimbal_command(std::shared_ptr<Gimbal> gimbal, int i)
  float pitch_deg = 30.0f * cosf(i / 360.0f * 2 * M_PI_F);
  float yaw_deg = 45.0f * sinf(i / 360.0f * 2 * M_PI_F);
  - gimbal->set_pitch_and_yaw_async(pitch_deg, yaw_deg,
  - std::bind(&receive_gimbal_result, _1));
  + gimbal->set_pitch_and_yaw_async(pitch_deg, yaw_deg, &receive_gimbal_result);
  }
  void send_gimbal_roi_location(std::shared_ptr<Gimbal> gimbal, double latitude_deg,
  double longitude_deg, float altitude_m)
  {
  gimbal->set_roi_location_async(latitude_deg, longitude_deg, altitude_m,
  - std::bind(&receive_gimbal_result, _1));
  + &receive_gimbal_result);
  }
  void receive_gimbal_result(Gimbal::Result result)
  