Remove wrongly backpatched piece of code in cube.c

Due to sloppy division of changes between f50c80dbb (which was not
back-patched) and 563a053bd, this piece of code was wrongly backpatched to
REL_10_STABLE and REL9_6_STABLE.  This code never causes real error because
its condition is never satisfied, but it's a dead code, which needs to be
removed.

Alexander Korotkov per gripe from Tom Lane

contrib/cube/cube.c

@@ -1587,7 +1587,6 @@ cube_coord_llur(PG_FUNCTION_ARGS)
 {
 	NDBOX	   *cube = PG_GETARG_NDBOX(0);
 	int			coord = PG_GETARG_INT32(1);
-	bool		inverse = false;
 	float8		result;
 
 	/* 0 is the only unsupported coordinate value */
@@ -1625,10 +1624,6 @@ cube_coord_llur(PG_FUNCTION_ARGS)
 		result = 0.0;
 	}
 
-	/* Inverse value if needed */
-	if (inverse)
-		result = -result;
-
 	PG_RETURN_FLOAT8(result);
 }