Fix the Makefile 'distclean' target (#811)

Without `-f`, running `make distclean` results in an error.

```
find . -name "*.pyc" -exec rm -v {} +
find . -name "*~" -exec rm -v {} +
find . -type d -name  "__pycache__" -exec rm -rv {} +
rm -rvf build dist MANIFEST .coverage .cache .pytest_cache htmlcov coverage.xml
rm -rvf tmp-test-dir-with-unique-name
rm -rvf baseline
rm -rvf result_images
rm -r *.egg-info
rm: *.egg-info: No such file or directory
make: *** [distclean] Error 1
```

Author: seisman

Makefile

@@ -63,4 +63,4 @@ clean:
 	rm -rvf result_images
 
 distclean: clean
-	rm -r *.egg-info
+	rm -rvf *.egg-info