release: Don't copy container images into /ftp/

These are installed into /ociimages/ and stage onto the download site
at /OCI-IMAGES/; they don't belong in the same directory as the
"distribution" .txz files.

Fixes:		d03c82c ("release: add optional OCI images")
MFC after:	1 minute
Sponsored by:	Amazon

release/Makefile

@@ -317,6 +317,7 @@ ftp: packagesystem
 	rm -rf ftp
 	mkdir -p ftp
 	cp *.txz MANIFEST ftp
+	rm -f ftp/container-*.txz
 
 release:	real-release vm-release cloudware-release oci-release
 	${MAKE} -C ${.CURDIR} ${.MAKEFLAGS} release-done