Revert discriminator changes introduced by #8606 (#10467)

Author: ArcturusZhang

specification/storagecache/resource-manager/Microsoft.StorageCache/stable/2019-11-01/storagecache.json

@@ -1434,6 +1434,7 @@
     "StorageTarget": {
       "description": "A storage system being cached by a Cache.",
       "type": "object",
+      "discriminator": "targetType",
       "properties": {
         "name": {
           "description": "Name of the Storage Target.",
@@ -1454,10 +1455,6 @@
           "x-ms-client-flatten": true,
           "description": "Properties of the Storage Target.",
           "type": "object",
-          "discriminator": "targetType",
-          "required": [
-            "targetType"
-          ],
           "properties": {
             "junctions": {
               "description": "List of Cache namespace junctions to target for namespace associations.",