c++: remove useless cast about user_token_number_max_

Reported by Frank Heckenbach.
https://lists.gnu.org/archive/html/bug-bison/2019-11/msg00016.html

The cast is needed when yytranslate_'s argument type is token_type,
i.e., when api.token.constructor is defined.

    373. types.at:138: testing lalr1.cc api.value.type=variant api.token.constructor ...
    ======== Testing with C++ standard flags: ''
    ../../tests/types.at:138: bison --color=no -fno-caret  -o test.cc test.y
    ../../tests/types.at:138: $CXX $CXXFLAGS $CPPFLAGS  $LDFLAGS -o test test.cc $LIBS
    stderr:
    test.cc:966:16: error: result of comparison of constant 257 with
                    expression of type 'yy::parser::token_type'
                   (aka 'yy::parser::token::yytokentype') is always true
                   [-Werror,-Wtautological-constant-out-of-range-compare]
        else if (t <= user_token_number_max_)
                 ~ ^  ~~~~~~~~~~~~~~~~~~~~~~
    1 error generated.

It is because it is expected that when api.token.constructor is
defined, only symbol constructors will be used, that yytranslate_ then
takes a token_type.  But it is wrong: we still allow literal
characters in this case, as demonstrated by test 373 for instance.

    %define api.value.type variant
    %define api.token.constructor
    %token <std::pair<int, int>> '1' '2';
    [...]
    static yy::parser::symbol_type yylex ()
    {
      static char const input[] = "12";
      int res = input[toknum++];
      typedef yy::parser::symbol_type symbol;
      if (res)
        return symbol (res, std::make_pair (res - '0', res - '0' + 1));
      else
        return symbol (res);
    }

So let yytranslate_ always take an int, which makes the cast truly
useless.

* data/skeletons/c++.m4, data/skeletons/lalr1.cc (yytranslate_): here.

data/skeletons/lalr1.cc

@@ -268,7 +268,9 @@ m4_define([b4_shared_declarations],
     static const ]b4_int_type(b4_table_ninf, b4_table_ninf)[ yytable_ninf_;
 
     /// Convert a scanner token number \a t to a symbol number.
-    static token_number_type yytranslate_ (]b4_token_ctor_if([token_type], [int])[ t);
+    /// In theory \a t should be a token_type, but character literals
+    /// are valid, yet not member of the token_type enum.
+    static token_number_type yytranslate_ (int t);
 
     // Tables.
 ]b4_parser_tables_declare[]b4_error_verbose_if([