Remove legacy upgrade script (#1744)

Related tiny-pilot/tinypilot-pro#1214

Based on our efforts to mitigate privilege escalation, the
`scripts/upgrade` script was identified as both tinypilot-writeable and
requiring root privileges. However, this script is now considered legacy
and can be removed because [it was only needed at a time when TinyPilot
could only be updated via the
terminal](https://codeapprove.com/pr/tiny-pilot/tinypilot/1743#thread-5433ea94-c337-4b07-a1cd-20651c032dd5).

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README.md

@@ -88,7 +88,7 @@ See the [CONTRIBUTING](CONTRIBUTING.md) file.
 To update to the latest version of TinyPilot, run the update script:
 
 ```bash
-/opt/tinypilot/scripts/upgrade && sudo reboot
+sudo /opt/tinypilot-privileged/scripts/update && sudo reboot
 ```
 
 ## Diagnostics

get-tinypilot.sh

@@ -44,7 +44,7 @@ if [[ "${HAS_PRO_INSTALLED}" = 1 ]]; then
     printf "Community Edition.\n\n"
     printf "You probably want to update to the latest version of TinyPilot "
     printf "Pro instead:\n\n"
-    printf "  /opt/tinypilot/scripts/upgrade && sudo reboot\n"
+    printf "  sudo /opt/tinypilot-privileged/scripts/update && sudo reboot\n"
     printf "\n"
     printf "If you *really* want to downgrade to TinyPilot Community Edition, "
     printf "type the following:\n\n"