Revert "grpc/compiler: Respect filename suffix and extension during code generation (#7343)" (#7406)

This reverts commit 97e89c5acd.

grpc/src/compiler/cpp_generator.h

@@ -33,8 +33,6 @@ struct Parameters {
   grpc::string grpc_search_path;
   // Generate GMOCK code to facilitate unit testing.
   bool generate_mock_code;
-  // By default, use "_generated.h"
-  std::string message_header_extension;
 };
 
 // Return the prologue of the generated header file.