I got the solution in the last version of the realsense2_camera.
The following configuration should be defined in order to achieve the PointCloud withou RGB:
<!-- Intel Realsense D435 driver -->
<include file="$(find realsense2_camera)/launch/rs_camera.launch">
<arg name="pointcloud_texture_stream" default="RS2_STREAM_ANY"/>
<arg name="enable_pointcloud" default="true"/>
<arg name="enable_color" default="false"/>
<arg name="enable_depth" default="true"/>
<arg name="enable_infra2" default="false"/>
<arg name="enable_infra1" default="false"/>
<arg name="align_depth" default="false"/>
<arg name="depth_width" default="640"/>
<arg name="depth_height" default="480"/>
</include>
Unfortunately the current version does not support the RS2_STREAM_DEPTH therefore the result is not the same as I wished cause when you choose the RS2_STREAM_ANY the PointCloud will have more than twice of Points which leads to have even a higher CPU consumption with respect to the PointCloud with RGB.