sched/clock: Remove CLOCK_MONOTONIC option from Kconfig

here is the reason:
1.clock_systime_timespec(core function) always exist regardless the setting
2.CLOCK_MONOTONIC is a foundamental clock type required by many places

Signed-off-by: Xiang Xiao <xiaoxiang@xiaomi.com>

fs/vfs/fs_epoll.c

@@ -387,12 +387,7 @@ int epoll_pwait(int epfd, FAR struct epoll_event *evs,
       expire.tv_sec  = timeout / 1000;
       expire.tv_nsec = timeout % 1000 * 1000;
 
-#ifdef CONFIG_CLOCK_MONOTONIC
       clock_gettime(CLOCK_MONOTONIC, &curr);
-#else
-      clock_gettime(CLOCK_REALTIME, &curr);
-#endif
-
       clock_timespec_add(&curr, &expire, &expire);
     }
 
@@ -403,11 +398,7 @@ again:
     }
   else
     {
-#ifdef CONFIG_CLOCK_MONOTONIC
       clock_gettime(CLOCK_MONOTONIC, &curr);
-#else
-      clock_gettime(CLOCK_REALTIME, &curr);
-#endif
       clock_timespec_subtract(&expire, &curr, &diff);
 
       rc = ppoll(eph->poll, eph->occupied + 1, &diff, sigmask);

fs/vfs/fs_timerfd.c

@@ -540,12 +540,9 @@ int timerfd_create(int clockid, int flags)
 
   /* Sanity checks. */
 
-  if (clockid != CLOCK_REALTIME
-#ifdef CONFIG_CLOCK_MONOTONIC
-      && clockid != CLOCK_MONOTONIC
-      && clockid != CLOCK_BOOTTIME
-#endif /* CONFIG_CLOCK_MONOTONIC */
-      )
+  if (clockid != CLOCK_REALTIME &&
+      clockid != CLOCK_MONOTONIC &&
+      clockid != CLOCK_BOOTTIME)
     {
       ret = -EINVAL;
       goto errout;