BUILD/MINOR: lua: fix a harmless build warning

BUILD/MINOR: lua: fix a harmless build warning

Dmitry Sivachenko reported the following build warning using Clang,
though it's harmless :

  src/hlua.c:1911:13: warning: variable '_socket_info_expanded_form' is not needed
        and will not be emitted [-Wunneeded-internal-declaration]
  static char _socket_info_expanded_form[] = SOCKET_INFO_EXPANDED_FORM;
              ^
Indeed, the variable is not used except to compute a sizeof which is
taken from the string it is initialized from. It probably is a leftover
after various code refactorings. Let's get rid of it now since it's not
used anymore.

No backport is needed.