mgmt: mcumgr: img_mgmt: Fix bug with old image state being present

This fixes an issue whereby the image state of an upload is present
even after an image erase command, which would instruct a client to
continue uploading at an offset that has no preceeding data.

Fixes #51842

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>
(cherry picked from commit 4c48b4f2)