`--dump=prism_parsetree` is no longer provided

Since it did not make sense without `--parser=prism` option, just a
duplication.  Now it is `--parser=prism --dump=parsetree`.

ruby.c

@@ -376,7 +376,6 @@ usage(const char *name, int help, int highlight, int columns)
         M("yydebug(+error-tolerant)",                "", "yydebug of yacc parser generator."),
         M("parsetree(+error-tolerant)",              "", "Abstract syntax tree (AST)."),
         M("parsetree_with_comment(+error-tolerant)", "", "AST with comments."),
-        M("prism_parsetree",                         "", "Prism AST."),
     };
     static const struct ruby_opt_message features[] = {
         M("gems",                  "", "Rubygems (only for debugging, default: "DEFAULT_RUBYGEMS_ENABLED")."),