docs/machine.Pin: Remove out_value() method.

This method isn't implemented in any port. It seemed to have originated
in cc3200 port, but actually never was implemented there either. In
general case, it's impossible to implement this method (for example, for
a perfect GPO, which has only output latch without any feedback look
into a CPU).

docs/library/machine.Pin.rst

@@ -146,12 +146,6 @@ Methods
 
    When setting the value this method returns ``None``.
 
-.. method:: Pin.out_value()
-
-   Return the value stored in the output buffer of a pin, regardless of its mode.
-
-   Not all ports implement this method.
-
 .. method:: Pin.__call__([x])
 
    Pin objects are callable.  The call method provides a (fast) shortcut to set