fix: Removing unittest.mock.patch from imports in test_charm.py

[Gmerold]

Description

Removes unused unittest.mock.patch from imports in test_charm.py

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that validate the behaviour of the software
• I validated that new and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have bumped the version of the library