%fixed-output-files: detach from %yacc

The name fixed-output-files is pretty clear: generate y.tab.c, as Yacc
does.  So let's detach this from %yacc which does more: it requires
POSIX Yacc behavior.

This directive is obsolete since December 29th 2001
8c9a50bee1.  It does not show in the
doc.  I don't want to spend more time on improving its diagnostics, it
could be removed just as well as far as I'm concerned.

* src/scan-gram.l, src/parse-gram.y (%fixed-output-files): Detach from
%yacc.

src/parse-gram.h

@@ -207,8 +207,6 @@ union GRAM_STYPE
   uniqstr PERCENT_NAME_PREFIX;
   /* "%pure-parser"  */
   uniqstr PERCENT_PURE_PARSER;
-  /* "%yacc"  */
-  uniqstr PERCENT_YACC;
   /* "[identifier]"  */
   uniqstr BRACKETED_ID;
   /* "identifier"  */