Remove Tauri desktop shell

Not going to be used — drop src-tauri/ and all references to it in
the docs. MapTool stays a plain Sinatra web app for now.

.gitignore

@@ -5,9 +5,6 @@
 /log/
 /tmp/
 
-# Rust / Tauri
-/src-tauri/target/
-
 # Offline map tile data (large binary; obtained/generated locally,
 # see docs/development.adoc)
 /data/*.pmtiles