runtime/protoimpl: remove ExtensionFieldsV1

This is no longer directly used by v1.

Change-Id: I14a283dd2d1572d50ebd82fdf55f7d86bfacc272
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/172438
Reviewed-by: Damien Neil <dneil@google.com>
diff --git a/runtime/protoimpl/impl.go b/runtime/protoimpl/impl.go
index bbfab27..617aed6 100644
--- a/runtime/protoimpl/impl.go
+++ b/runtime/protoimpl/impl.go
@@ -46,6 +46,5 @@
 	UnknownFields   = []byte
 	SizeCache       = int32
 
-	ExtensionFieldV1  = impl.ExtensionFieldV1
-	ExtensionFieldsV1 = impl.ExtensionFieldsV1
+	ExtensionFieldV1 = impl.ExtensionFieldV1
 )