Fix compiler warnings and remove obsolete files

The extension should now compile with
 make CFLAGS="-Wall -Wextra -Werror -Wno-unused-parameter -Wno-sign-compare -g -O2"

Removed ustring.c, luasandbox_unicode.h, and m4/ac_check_icu.m4, as they
have been unused since Aug 2012.

Change-Id: I63998c026ac15c96bbfb72b0fa3a770aae1c0ba9