[PL/Output] Rm 'process_id' from out file names.

Documentation/ProjectFile/prj/time_loop/output/t_prefix.md

 Template for the prefix which will be prepended to the output files. Allowed
 template expressions are:
 - {:meshname}
-- {:process_id}
 - {:timestep}
 - {:time}.
 
-Default value: {:meshname}{:process_id}
+Default value: {:meshname}
 
 Remark: The name of the pvd-file will be constructed out of the prefix.
 

ProcessLib/Output/CreateOutput.cpp

@@ -52,7 +52,7 @@ std::unique_ptr<Output> createOutput(
     auto const prefix =
         //! \ogs_file_param{prj__time_loop__output__prefix}
         config.getConfigParameter<std::string>("prefix",
-                                               "{:meshname}{:process_id}");
+                                               "{:meshname}");
 
     auto const suffix =
         //! \ogs_file_param{prj__time_loop__output__suffix}