Remove git_time_monotonic

`git_time_monotonic` was added so that non-native bindings like rugged
could get high-resolution timing for benchmarking.  However, this is
outside the scope of libgit2 *and* rugged decided not to use this
function in the first place.

Google suggests that absolutely _nobody_ is using this function and we
don't want to be in the benchmarking business.  Remove the function.

Author: ethomson

include/git2/sys/time.h

@@ -864,11 +864,6 @@ int git__utf8_iterate(const uint8_t *str, int str_len, int32_t *dst)
 	return length;
 }
 
-double git_time_monotonic(void)
-{
-	return git__timer();
-}
-
 size_t git__utf8_valid_buf_length(const uint8_t *str, size_t str_len)
 {
 	size_t offset = 0;