Remove unused function

Building with clang warns:

tests/posix/timers/src/clock.c:49:20: error: unused function
'tp_diff_in_range_ns' [-Werror,-Wunused-function]
static inline bool tp_diff_in_range_ns(const struct timespec *a, const struct timespec *b,
                   ^

tests/posix/timers/src/clock.c

@@ -45,15 +45,6 @@ static inline int64_t ts_to_ns(const struct timespec *ts)
 _decl_op(bool, tp_ge, >=);     /* a >= b */
 _decl_op(int64_t, tp_diff, -); /* a - b */
 
-/* lo <= (a - b) < hi */
-static inline bool tp_diff_in_range_ns(const struct timespec *a, const struct timespec *b,
-				       int64_t lo, int64_t hi)
-{
-	int64_t diff = tp_diff(a, b);
-
-	return diff >= lo && diff < hi;
-}
-
 ZTEST(posix_timers, test_clock_gettime)
 {
 	struct timespec ts;