Reapply "Remove service client for privatenetworks (boto#3474)" (boto#3475) (boto#3476)

This reverts commit 69473f5.

Author: SamRemis

.changes/next-release/api-change-privatenetworks-24583.json

@@ -0,0 +1,5 @@
+{
+  "type": "api-change",
+  "category": "``privatenetworks``",
+  "description": "The privatenetworks client has been removed following the deprecation of the service."
+}

botocore/data/endpoints.json

@@ -18875,13 +18875,6 @@
           }
         }
       },
-      "private-networks" : {
-        "endpoints" : {
-          "us-east-1" : { },
-          "us-east-2" : { },
-          "us-west-2" : { }
-        }
-      },
       "profile" : {
         "endpoints" : {
           "af-south-1" : { },