Remove default Janus systemd service (#23)

Related tiny-pilot/tinypilot#1917

This PR skips the installation of a Janus systemd service because Janus
is only installed with sample config files which still needs to be
configured in order to startup correctly.

Use the following draft PR to test this code on device:
- tiny-pilot/tinypilot#1918

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Author: jdeanwallace