Drop the use of the uStreamer port Ansible variable

In https://github.com/tiny-pilot/tinypilot/pull/1562, we're moving uStreamer from port 8001 to port 48001. However, before we do that I thought it might be a good time to hardcode the port number and ~~enforce non-user-configurability~~ drop the use of the ustreamer_port Ansible variable.

According to https://github.com/tiny-pilot/tinypilot-pro/issues/972, ustreamer_port is already considered a non-user-configurable variable. This PR enforces ustreamer_port as being non-user-configurable.

This PR does the following:

Reverts https://github.com/tiny-pilot/tinypilot/pull/1493 (which introduced that _CONSTANTS variable in app/update/settings.py)
Hardcodes the uStreamer port to 8001 in the TinyPilot NGINX config and the uStreamer launcher script.
Drops the use of the ustreamer_port Ansible variable.
Cleans up the /home/tinypilot/settings.yml file by removing any user-defined ustreamer_port at install/update time.

Notes

TinyPilot Pro would require the following additional change:

# debian-pkg/debian/tinypilot.postinst

 KEYBOARD_PATH = '{{ tinypilot_keyboard_interface }}'
 MOUSE_PATH = '{{ tinypilot_mouse_interface }}'
-USTREAMER_BASEURL = 'http://127.0.0.1:{{ ustreamer_port }}'
+USTREAMER_BASEURL = 'http://127.0.0.1:8001'

tiny-pilot / tinypilot

Drop the use of the uStreamer port Ansible variable #1563

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