NetworkPkg/DnsDxe: Remove unnecessary NULL pointer check.

NetworkPkg/DnsDxe: Remove unnecessary NULL pointer check.

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1469

Since the value of ItemCache4/ItemCache6 is retrieved from the list
Entry, it can't be the NULL pointer, so just remove the unnecessary
check.

Cc: Ye Ting <ting.ye@intel.com>
Cc: Fu Siyuan <siyuan.fu@intel.com>
Cc: Wu Hao A <hao.a.wu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Wu Jiaxin <jiaxin.wu@intel.com>
Reviewed-by: Wu Hao A <hao.a.wu@intel.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>