[noetic] Gazebo Dependency Fixes

Add Stretch Gazebo Support for Noetic

Removes two IMU launch files. They are not used by other
stretch_ros code, and they appear to not function properly.
For example, they do not appear to use a proper tf frame for
the IMU, can produce problematic tf trees, and can result
in tf query timing problems.

The demo launch files started IMU nodes that were
unused, produced warnings, and created problematic tf trees.
With this commit, they no longer start these nodes.
Quick tests of some of the demos indicate that they
work as well as they did before. However, I did not test
hello_world.launch.

FUNMAP obstacle detection failed due to the frustum of the
RGB colored 3D points not overlapping with the region in front
of the robot that is required to detect the floor when moving
forward. Since the IR camera 3D point frustum still overlaps
with the region, it seems that an upstream Realsense D435i
change may be causing only the RGB colored 3D points to be used
now.

For now, this is a quick fix that makes the robot look down lower
when moving forward, so that the color camera frustum overlaps
with the obstacle detection region.

The FUNMAP mapping launch file started IMU nodes that were
unused, produced warnings, and created a problematic tf tree.
With this commit, mapping.launch no longer starts these nodes.
Mapping and navigation continued to function in a brief test
and the IMU related warnings were no longer produced.

I used the following command for testing.

$ roslaunch stretch_funmap mapping.launch