Rename 'expiration.on-sync' to 'purge.on-sync' (#3556)

Taskwarrior uses "expire" to refer to deletion of tasks past their
"until" date, so let's use `purge` to link this semantically to the
`task purge` command.

test/CMakeLists.txt

@@ -111,7 +111,6 @@ set (pythonTests
   encoding.test.py
   enpassant.test.py
   exec.test.py
-  expiration.test.py
   export.test.py
   feature.559.test.py
   feature.default.project.test.py