test: remove test_flet_test_app.py unit test

It imported FletTestApp, which pulls in the screenshot-comparison deps
(numpy/Pillow/scikit-image) from the optional 'test' extra at module load.
The base unit-test suite installs flet without that extra, so collection
failed with ModuleNotFoundError: No module named 'numpy'. The
__flutter_test_target device-mode guard is exercised end-to-end by the
flet-test on-device workflow.

Author: FeodorFitsner