Revert "DebugUsb3: Check mUsb3Instance before dereferencing it"

This reverts commit 6ef394ffe2.

Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Hao Wu <hao.a.wu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Hao Wu <hao.a.wu@intel.com>
(cherry picked from commit abea3bca82)

SourceLevelDebugPkg/Library/DebugCommunicationLibUsb3/DebugCommunicationLibUsb3Dxe.c

@@ -443,7 +443,7 @@ DebugCommunicationUsb3DxeConstructor (
   }
 
 Done:
-  if ((mUsb3Instance != NULL) && mUsb3Instance->Ready && (mUsb3Instance->PciIoEvent == 0)) {
+  if (mUsb3Instance->Ready && (mUsb3Instance->PciIoEvent == 0)) {
     Status = Usb3NamedEventListen (
                &gEfiPciIoProtocolGuid,
                TPL_NOTIFY,