Remove outdated time.monotonic reference (GH-13264)

Per ae586497, time.monotonic is always available, making the old note outdated.

Doc/library/sched.rst

@@ -20,8 +20,7 @@ scheduler:
    The :class:`scheduler` class defines a generic interface to scheduling events.
    It needs two functions to actually deal with the "outside world" --- *timefunc*
    should be callable without arguments, and return  a number (the "time", in any
-   units whatsoever). If time.monotonic is not available, the *timefunc* default
-   is time.time instead. The *delayfunc* function should be callable with one
+   units whatsoever).  The *delayfunc* function should be callable with one
    argument, compatible with the output of *timefunc*, and should delay that many
    time units. *delayfunc* will also be called with the argument ``0`` after each
    event is run to allow other threads an opportunity to run in multi-threaded