GCS_MAVLink: add do_disarm_checks boolean to disarm call

this creates symmetry between arming and disarming, at least as far as
the top-level arm() and disarm() calls are concerned.

libraries/GCS_MAVLink/GCS.h

@@ -511,8 +511,6 @@ protected:
     static constexpr const float magic_force_arm_value = 2989.0f;
     static constexpr const float magic_force_disarm_value = 21196.0f;
 
-    virtual bool allow_disarm() const { return true; }
-
     void manual_override(RC_Channel *c, int16_t value_in, uint16_t offset, float scaler, const uint32_t tnow, bool reversed = false);
 
     /*

libraries/GCS_MAVLink/GCS_Common.cpp

@@ -3864,13 +3864,9 @@ MAV_RESULT GCS_MAVLINK::handle_command_component_arm_disarm(const mavlink_comman
         if (!AP::arming().is_armed()) {
             return MAV_RESULT_ACCEPTED;
         }
-        // allow vehicle to disallow disarm.  Copter does this if
-        // the vehicle isn't considered landed.
-        if (!allow_disarm() &&
-            !is_equal(packet.param2, magic_force_disarm_value)) {
-            return MAV_RESULT_FAILED;
-        }
-        if (AP::arming().disarm(AP_Arming::Method::MAVLINK)) {
+        const bool forced = is_equal(packet.param2, magic_force_disarm_value);
+        // note disarm()'s second parameter is "do_disarm_checks"
+        if (AP::arming().disarm(AP_Arming::Method::MAVLINK, !forced)) {
             return MAV_RESULT_ACCEPTED;
         }
         return MAV_RESULT_FAILED;