openssh: strip openssh-sftp-server dependencies, notably openssl

openssh-sftp-server is actually a tiny binary that has nearly no dependencies:
$ mips-openwrt-linux-objdump -x sftp-server | grep NEEDED
  NEEDED               libgcc_s.so.1
  NEEDED               libc.so.0

Stripping its DEPENDS allows building a system with SFTP support, without
pulling in OpenSSL.

net/openssh/Makefile

@@ -121,6 +121,7 @@ endef
 define Package/openssh-sftp-server
 	$(call Package/openssh/Default)
 	TITLE+= SFTP server
+	DEPENDS:=
 endef
 
 define Package/openssh-sftp-server/description