Remove broken mirrors, prefer HTTP over FTP.

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.

Signed-off-by: Peter Wagner <tripolar@gmx.at>