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: Richard Weinberger <richard@nod.at>