release: Remove empty suffix from file names

For "release" builds (as opposed to "snapshot" builds -- in this
context BETAs and RCs are "releases") ${SNAP_SUFFIX} is empty; but it
stuck into some ociimages filenames via a copy-and-paste error.

The final filenames on the download mirrors were not affected, so
this does not need to be merged to releng/14.2.

MFC after: 3 days
Sponsored by: Amazon