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>
diff --git a/boot/zephyr/Kconfig b/boot/zephyr/Kconfig
index a601242..99b5a5c 100644
--- a/boot/zephyr/Kconfig
+++ b/boot/zephyr/Kconfig
@@ -663,10 +663,6 @@
 	default n if SOC_FAMILY_NRF
 	default y
 
-config LOG_IMMEDIATE
-	default n if MULTITHREADING
-	default y
-
 config LOG_PROCESS_THREAD
 	default n # mcuboot has its own log processing thread