openssh-server: deprecate the ecdsa HostKey

openssh-server: deprecate the ecdsa HostKey

The init.d script for sshd never generates an ecdsa HostKey as seen
here:

	for type in rsa ed25519
	do
		# check for keys
		key=/etc/ssh/ssh_host_${type}_key
		[ ! -f $key ] && {
			# generate missing keys
			[ -x /usr/bin/ssh-keygen ] && {
				/usr/bin/ssh-keygen -N '' -t $type -f $key 2>&- >&-
			}
		}
	done

so we'll never succeed at loading one.  Get rid of the resultant
error message in logging:

May  5 17:13:59 OpenWrt sshd[20070]: error: Unable to load host key: /etc/ssh/ssh_host_ecdsa_key

Signed-off-by: Philip Prindeville <philipp@redfish-solutions.com>