Drop getCommandStr member of Process

Formatting the merged command string is now implemented in an platform
independent way. Drop the Process member getCommandStr designed for
overrides of individual platforms.

unsupported/UnsupportedProcess.c

@@ -100,6 +100,5 @@ const ProcessClass UnsupportedProcess_class = {
       .compare = Process_compare
    },
    .writeField = UnsupportedProcess_writeField,
-   .getCommandStr = NULL,
    .compareByKey = UnsupportedProcess_compareByKey
 };