c++: get rid of symbol_type::token ()

It is not used.  And its implementation was wrong when api.token.raw
was defined, as it was still mapping to the external token numbers,
instead of the internal ones.  Besides it was provided only when
api.token.constructor is defined, yet always declared.

* data/skeletons/c++.m4 (by_type::token): Remove, useless.

TODO

@@ -135,9 +135,7 @@ $ ./tests/testsuite -l | grep errors | sed q
 
 * Short term
 ** Get rid of YYPRINT and b4_toknum
-
-** C++ a closer look at parser::by_type::token
-It can certainly be simplified with we use api.token.raw.
+Besides yytoknum is wrong when api.token.raw is defined.
 
 ** Stop indentation in diagnostics
 Before Bison 2.7, we printed "flatly" the dependencies in long diagnostics: