test: remove unused yaml import

We import yaml in conftest, but it's not used. This leads to errors
like
```
mender_integration/tests/conftest.py:20: in <module>
    import yaml
E   ModuleNotFoundError: No module named 'yaml'
```
because we don't have yaml as a python-requirement

Signed-off-by: Daniel Skinstad Drabitzius <[email protected]>

Author: danielskinstad

tests/conftest.py

@@ -17,7 +17,6 @@
 import subprocess
 import shutil
 import tempfile
-import yaml
 import packaging.version
 
 import multiprocessing