OvmfPkg: correct the IScsiDxe module included for the IPv6 stack

OvmfPkg: correct the IScsiDxe module included for the IPv6 stack

Always use IScsiDxe from NetworkPkg when IPv6 is enabled since it provides
the complete ISCSI support.

NOTE: This makes OpenSSL a hard requirement when NETWORK_IP6_ENABLE is
      true.

(Based on Jiaxin's suggestion)

Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Justen Jordan L <jordan.l.justen@intel.com>
Cc: Wu Jiaxin <jiaxin.wu@intel.com>
Cc: Long Qin <qin.long@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Gary Lin <glin@suse.com>
Reviewed-by: Wu Jiaxin <jiaxin.wu@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
[lersek@redhat.com: update subject line]
Signed-off-by: Laszlo Ersek <lersek@redhat.com>