man-pages: Don't have a separate docdev output

I got:

$ nix-env -f . -iA manpages
$ man mmap
No manual entry for mmap

which is suboptimal for a package that "documents the Linux kernel and
C library interfaces that are employed by user-space programs"
(https://www.kernel.org/doc/man-pages/).

pkgs/data/documentation/man-pages/default.nix

···
     sha256 = "1vimj3va16plxmv46rw6nzw4m9l11hb7r1d217y1bjpd5q8nw8qz";
   };
 
-  # keep developer docs separately (man2 and man3)
-  outputs = [ "out" "docdev" ];
   makeFlags = [ "MANDIR=$(out)/share/man" ];
-  postFixup = ''
-    moveToOutput share/man/man2 "$docdev"
-  '';
 
   meta = with stdenv.lib; {
     description = "Linux development manual pages";