zephyr: do not set defaults for LOG_IMMEDIATE Kconfig

LOG_IMMEDIATE Kconfig option has been repurposed and is now a Zephyr's
internal, non-visible symbol that should not be used by the end user.

The logging mode used by MCUBoot is defined in prj.conf

Signed-off-by: Piotr Mienkowski <piotr.mienkowski@gmail.com>