staging: greybus: drop loopback test files

The greybus loopback test tool does not belong burried down in a
driver-specific directory.  If it is needed, it should be somewhere
else, like in the testing directory.  But as the loopback driver is
probably never going to be merged out of the staging directory, let's
just delete the test alltogether for now.  If it's needed in the future,
it can be brought back with a revert.

Also, having an Android.mk file in the kernel source tree breaks some
Android build systems when trying to build from a read-only source tree,
the report of which prompted this being a good reason to remove it as
well.

Cc: Johan Hovold <johan@kernel.org>
Cc: Jack Schofield <schofija@oregonstate.edu>
Cc: Vaibhav Nagare <vnagare@redhat.com>
Cc: greybus-dev@lists.linaro.org
Acked-by: Alex Elder <elder@linaro.org>
Link: https://lore.kernel.org/r/2023040613-huntsman-goldsmith-4a41@gregkh


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>