tests: bluetooth: disable CDC ACM logging

Disable CDC ACM logging in configuraton overlay used in test case
bluetooth.shell.cdc_acm.
The build warning was introduced by the commit e8c48678
("usb: cdc_acm: disable logging if used for shell with logging")
to preven recursive logging when using the CDC ACM UART as shell or
logging backend

Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>