Clean up after building Cpp FMU (the sources are saved inside the FMU)

Belonging to [master]:
  - #2156
  - OpenModelica/OpenModelica-testsuite#838

Author: rfranke

Compiler/Script/CevalScriptBackend.mo

@@ -3417,6 +3417,7 @@ algorithm
       end if;
       ExecStat.execStat("buildModelFMU: Generate C++ for platform " + platform);
     end for;
+    System.systemCall("make -f " + filenameprefix + "_FMU.makefile clean");
     return;
   end if;