tests: Bluetooth: df: remove unnecessary testcase.yaml
tests: Bluetooth: df: remove unnecessary testcase.yaml

The testcase.yaml file in tests/bluetooth/df directory makes
twister trying to build tests that are not available.
All tests in the df subdirectory have private testcase.yaml
files.

Signed-off-by: Piotr Pryga <4cae3388a6c6e480df2109bb21aa832bf2c59595@nordicsemi.no>

finikorg/zephyr,galak/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr