Remove obsolete and confusing comment

The comment was added in commit cfdb196
to explain why Rcpp_list6() is special.
However, in a later commit 6eac444
Rcpp_list{2,3,4,5}() were added with similar definitions.
This means that Rcpp_list6() is no longer special, and the comment is now
confusing rather than informative.

Author: ellert

ChangeLog

@@ -1,3 +1,7 @@
+2020-08-20  Mattias Ellert  <mattias.ellert@physics.uu.se>
+
+        * inst/include/Rcpp/lang.h: Remove obsolete and confusing comment
+
 2020-08-05  Dirk Eddelbuettel  <edd@debian.org>
 
         * DESCRIPTION (Version, Date): Roll minor version

inst/include/Rcpp/lang.h

@@ -61,11 +61,7 @@ inline SEXP Rcpp_list5(SEXP x0, SEXP x1, SEXP x2, SEXP x3, SEXP x4) {
     return x0;
 }
 
-
-
-// `Rf_lang6()` is available on R 3.3, but `Rf_list6()` is not
-inline SEXP Rcpp_list6( SEXP x0, SEXP x1, SEXP x2, SEXP x3, SEXP x4, SEXP x5 )
-{
+inline SEXP Rcpp_list6(SEXP x0, SEXP x1, SEXP x2, SEXP x3, SEXP x4, SEXP x5) {
     PROTECT(x0);
     x0 = Rf_cons(x0, Rcpp_list5(x1, x2, x3, x4, x5));
     UNPROTECT(1);