Remove the --mm option from goto-diff

This was tested for but does not appear as an option in
goto_diff_parse_options.h as a valid option so this code can never be used.
Worse, goto-diff doesn't include any of the code that uses it, nor can I
think what the memory model might have to do with diffing goto-programs.

Author: martin

src/goto-diff/goto_diff_parse_options.cpp

@@ -70,9 +70,6 @@ void goto_diff_parse_optionst::get_command_line_options(optionst &options)
   if(cmdline.isset("cover"))
     parse_cover_options(cmdline, options);
 
-  if(cmdline.isset("mm"))
-    options.set_option("mm", cmdline.get_value("mm"));
-
   // all checks supported by goto_check
   PARSE_OPTIONS_GOTO_CHECK(cmdline, options);