logging: backends: rtt: Do not enable when shell logging is used

Typically, when shell logging backend is used user wants to get
logs on shell instances so do not enable raw RTT log backend in
that case. When raw backend is enabled then initial logs go to
RTT backend and they are not visible on shell which starts later
as it requires additional thread.

It can be assumed that when RTT backend is used it will be
explicitly enabled by the user.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>