docs/library/index: Remove all conditionals from library index.

It's fair to just provide a link to all available modules, regardless of
the port.  Most of the existing ports (unix, stm32, esp8266, esp32) share
most of the same set of modules anyway, so no need to maintain separate
lists for them.  And there's a big discussion at the start of this index
about modules not being available on a given port.

For port-specific modules, they can also be listed unconditionally because
they have headings that explicitly state they are only available on certain
ports.

docs/library/index.rst

@@ -65,8 +65,6 @@ For example, ``import json`` will first search for a file ``json.py`` (or packag
 directory ``json``) and load that module if it is found.  If nothing is found,
 it will fallback to loading the built-in ``ujson`` module.
 
-.. only:: port_unix
-
 .. toctree::
    :maxdepth: 1
 
@@ -93,77 +91,6 @@ it will fallback to loading the built-in ``ujson`` module.
    uzlib.rst
    _thread.rst
 
-.. only:: port_pyboard
-
-    .. toctree::
-       :maxdepth: 1
-
-       builtins.rst
-       array.rst
-       cmath.rst
-       gc.rst
-       math.rst
-       sys.rst
-       ubinascii.rst
-       ucollections.rst
-       uerrno.rst
-       uhashlib.rst
-       uheapq.rst
-       uio.rst
-       ujson.rst
-       uos.rst
-       ure.rst
-       uselect.rst
-       usocket.rst
-       ustruct.rst
-       utime.rst
-       uzlib.rst
-       _thread.rst
-
-.. only:: port_wipy
-
-    .. toctree::
-       :maxdepth: 1
-
-       builtins.rst
-       array.rst
-       gc.rst
-       sys.rst
-       ubinascii.rst
-       ujson.rst
-       uos.rst
-       ure.rst
-       uselect.rst
-       usocket.rst
-       ussl.rst
-       utime.rst
-
-.. only:: port_esp8266
-
-    .. toctree::
-       :maxdepth: 1
-
-       builtins.rst
-       array.rst
-       gc.rst
-       math.rst
-       sys.rst
-       ubinascii.rst
-       ucollections.rst
-       uerrno.rst
-       uhashlib.rst
-       uheapq.rst
-       uio.rst
-       ujson.rst
-       uos.rst
-       ure.rst
-       uselect.rst
-       usocket.rst
-       ussl.rst
-       ustruct.rst
-       utime.rst
-       uzlib.rst
-
 
 MicroPython-specific libraries
 ------------------------------
@@ -183,8 +110,6 @@ the following libraries.
    uctypes.rst
 
 
-.. only:: port_pyboard
-
 Libraries specific to the pyboard
 ---------------------------------
 
@@ -196,10 +121,9 @@ the following libraries.
   pyb.rst
   lcd160cr.rst
 
-.. only:: port_wipy
 
 Libraries specific to the WiPy
-   ---------------------------------
+------------------------------
 
 The following libraries are specific to the WiPy.
 
@@ -209,8 +133,6 @@ the following libraries.
   wipy.rst
 
 
-.. only:: port_esp8266
-
 Libraries specific to the ESP8266
 ---------------------------------