logging: Fix LOG_OUTPUT dependency

When CONFIG_LOG_OUTPUT is set, it indicates that logging strings
are formatted by the application (using log_output module). It is
not needed when backend works in the dictionary mode. So far
LOG_OUTPUT was set also when dictionary mode was used and that
prevented removing of the logging strings from binary which is
an important feature of the dictionary logging.

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