um: Remove unused timeval_to_ns() function

The timeval_to_ns() function doesn't appear to be used anywhere, as far
as I (or git grep) can tell, and clang throws up a warning about it:

../arch/um/os-Linux/time.c:21:25: warning: unused function 'timeval_to_ns' [-Wunused-function]
static inline long long timeval_to_ns(const struct timeval *tv)
                        ^
1 warning generated.

Signed-off-by: David Gow <davidgow@google.com>
Acked-By: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220224081233.1765788-1-davidgow@google.com

arch/um/os-Linux/time.c

@@ -18,12 +18,6 @@
 
 static timer_t event_high_res_timer = 0;
 
-static inline long long timeval_to_ns(const struct timeval *tv)
-{
-	return ((long long) tv->tv_sec * UM_NSEC_PER_SEC) +
-		tv->tv_usec * UM_NSEC_PER_USEC;
-}
-
 static inline long long timespec_to_ns(const struct timespec *ts)
 {
 	return ((long long) ts->tv_sec * UM_NSEC_PER_SEC) + ts->tv_nsec;