zephyr: Do not build in debug mode

Builds in the default zephyr mode (optimised for size) which
saves about 6KiB of flash on a default nrf52840dk build.

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>
diff --git a/boot/zephyr/prj.conf b/boot/zephyr/prj.conf
index 8d3cdb8..a6da049 100644
--- a/boot/zephyr/prj.conf
+++ b/boot/zephyr/prj.conf
@@ -1,4 +1,3 @@
-CONFIG_DEBUG=y
 CONFIG_PM=n
 
 CONFIG_MAIN_STACK_SIZE=10240