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.