Remove "optional" keyword in proto3 test file.

protoc will soon be rejecting this keyword in proto3 files.

protoc-gen-go/testdata/proto3.proto

@@ -40,9 +40,9 @@ message Request {
     UMAMI = 2;
     GOPHERLICIOUS = 3;
   }
-  string name = 1;  // "optional" may be omitted
+  string name = 1;
   repeated int64 key = 2;
-  optional Flavour taste = 3;
+  Flavour taste = 3;
   Book book = 4;
 }