tests: Bluetooth: df: remove duplicated code from CTE req tests

There is a common implementation of connection handling code
that may be shared by tests: connection_cte_req and connection_cte-
tx_params.
This commit removes code implemented in common.c and common.h
files. The same code is currently available in ../df/common/
directory.
All calls were updated to use function from common implementation.

Signed-off-by: Piotr Pryga <piotr.pryga@nordicsemi.no>