Supposing you're on the Active MPU...is the command:
delete /unreserved flash:/a10500-cmw520-r1126p02.bin
unable to permanently delete (without involving the system's recycle-bin) the a10500-cmw520-r1126p02.bin file?
Note (from Documentation):
The delete /unreserved file-url command deletes a file permanently and the action cannot be undone.
The delete file-url command moves a file to the recycle bin and the file still occupies storage space. To
permanently delete the file from the recycle bin execute the reset recycle-bin command in the original
directory of the file.
The HPE FlexNetwork 10500 Switch Series provides very good documentation (e.g. Release Notes) about managing its system's files.