rbd-mirror: fix bootstrap sequence while the image is removed

rbd-mirror: fix bootstrap sequence while the image is removed

If the image is being removed the PrepareRemoteImageRequest was
returning the same error if the image was disabled or non primary which
doesn't allow the BootstrapRequest to have the correct error handling.

This commit fix this behavior by considering that the remote image is
already deleted if the image is in disabling state.

Signed-off-by: Arthur Outhenin-Chalandre <arthur.outhenin-chalandre@cern.ch>
(cherry picked from commit ff60aec2d9efa1842383ba0a5c3bd6b5a29389c6)