linux: ignore fractional time in uv_uptime()

Before this commit, uv_uptime() returned the nanoseconds as the
fractional part of the uptime.  It's a bit inconsistent with the
output on other platforms because those only return whole seconds
(with the possible exception of Windows.)

This commit changes linux-core.c to only return whole seconds.

src/unix/linux-core.c

@@ -368,7 +368,6 @@ int uv_uptime(double* uptime) {
     return -errno;
 
   *uptime = now.tv_sec;
-  *uptime += (double)now.tv_nsec / 1000000000.0;
   return 0;
 }