Unused variable error when compiling with extra errors

[ozhang05]

include/sol/stack_check_unqualified.hpp:585:75: Warning Unused Variable 'indextype'

Very minor error, but when compiling with extra error flags (-Wall -Wextra -Werror) reveals an unused variable error at line 585. Seemed to fix it for me by deleting the variable (unsure if this breaks other things), or just adding a (void) indextype.

[ysmolski]

It is safe to change signature of the function to this:

		static bool check_two(types<K, V>, lua_State* arg_L, int relindex, type, Handler&& handler, record& tracking) {

indextype is not used in the function check_two as in the check_one. I am too lazy to make a PR, feel free to make this simple PR.