Remove pytest config 'testspath'

pytest is logging a warning about this being unknown:

```
/.../_pytest/config/__init__.py:1233: PytestConfigWarning: Unknown config option: testspath
```

I can't find any reference to it in the pytest docs or changelog.

Author: adamchainz

setup.cfg

@@ -3,7 +3,6 @@ license_file = LICENSE.md
 
 [tool:pytest]
 addopts=--tb=short --strict -ra
-testspath = tests
 
 [flake8]
 ignore = E501,W504