Remove Snowboy integration.

It hurts to see it go, as I really believed in this project.

But the website of the project went away in 2020, the Github project
hasn't seen any activity since 2021, and the fork that is supposed to be
used as a replacement for training .pmdl models hasn't been updated
since 2021 - and it only supports Python 2 on Ubuntu 16.04 or 18.04.

One day I may dedicate some efforts to bring Snowboy back to life, but
until then it's definitely not in a state where it's usable for a
Platypush integration.

docs/source/backends.rst

@@ -8,7 +8,6 @@ Backends
 
     platypush/backend/adafruit.io.rst
     platypush/backend/alarm.rst
-    platypush/backend/assistant.snowboy.rst
     platypush/backend/button.flic.rst
     platypush/backend/camera.pi.rst
     platypush/backend/chat.telegram.rst

docs/source/platypush/backend/assistant.snowboy.rst

@@ -1,6 +0,0 @@
-``assistant.snowboy``
-=======================================
-
-.. automodule:: platypush.backend.assistant.snowboy
-    :members:
-