Removed unused GraphicalUnitTester (#3054)

* Removed graphicalUnitTester

* Removed reference about it in teh docs

Author: huguesdevimeux

docs/source/contributing/testing.rst

@@ -112,7 +112,6 @@ The ``manim/tests`` directory looks like this:
     │   └── test_cli_flags.py
     └── utils
         ├── commands.py
-        ├── GraphicalUnitTester.py
         ├── __init__.py
         ├── testing_utils.py
         └── video_tester.py