Fix Descent 1 robots-drop-robots logic

Commit 7a183c1 ("Use enum class for robot_id") made an incorrect
change to the Descent 1 call to `robot_create`, causing the game to drop
the wrong type of robot.  Descent 2 was not affected.  This commit both
fixes the incorrect type, which is readily visible, and fixes the
created robot's radius, which Descent 1 has always done wrong and
Descent 2 has always done right.

The second argument to `robot_create` is the ID of the robot to create
(Hulk, Lifter, Spider, etc.).  Prior to 7a183c1, the second
argument was `id`, which came from the caller and was of type `int`.
Both games had a related variable `robot_id` used for lookup in the
`Robot_info` array.  In Descent 1, `robot_id` was set from a cast of
`ObjId[type]`.  In Descent 2, `robot_id` was set from a cast of `id`.

Based on the observation that `Robot_info[robot_id]` was used to read
`.model_num`, I believed that `robot_id` was suitable for use as the ID
of the robot to create.  For Descent 2, this was true.  For Descent 1,
it was not true.  Based on that belief, commit 7a183c1 changed the
second parameter from `id` to `robot_id` (which was also renamed to
`rid`).  For Descent 1, this caused the wrong robot to drop.  For
Descent 2, this was a no-op change.

Reported-by: JeodC <dxx-rebirth#742>
Bisected-by: InfernalCore <dxx-rebirth#742 (comment)>
Fixes: 7a183c1 ("Use enum class for robot_id")

similar/main/fireball.cpp

@@ -1197,14 +1197,7 @@ static bool drop_robot_egg(const d_robot_info_array &Robot_info, const contained
 //				new_pos.y += (d_rand()-16384)*7;
 //				new_pos.z += (d_rand()-16384)*6;
 
-#if defined(DXX_BUILD_DESCENT_I)
-				/* ObjId appears to serve as both a polygon_model_index and as
-				 * a robot index.
-				 */
-				const auto rid = static_cast<robot_id>(ObjId[underlying_value(type)]);
-#elif defined(DXX_BUILD_DESCENT_II)
 				const auto rid = static_cast<robot_id>(id);
-#endif
 				auto &robptr = Robot_info[rid];
 				const auto &&objp = robot_create(Robot_info, rid, segnum, new_pos, &vmd_identity_matrix, Polygon_models[robptr.model_num].rad, ai_behavior::AIB_NORMAL);
 				if (objp == object_none)