zephyr: removed CONFIG_SYSTEM_CLOCK_DISABLE=y

CONFIG_SYSTEM_CLOCK_DISABLE property was removed from
zephyr codebase. It is not needed anymore since the empty
sys_clock_disable() callback is provided if the platform
doesn't support system clock disable capability.

Reference:
https://github.com/zephyrproject-rtos/zephyr/commit/78dc8ce33828b1d995d520751fe761ade6103155

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>
diff --git a/boot/zephyr/boards/nrf52_minimal_footprint.conf b/boot/zephyr/boards/nrf52_minimal_footprint.conf
index f223f12..c315b44 100644
--- a/boot/zephyr/boards/nrf52_minimal_footprint.conf
+++ b/boot/zephyr/boards/nrf52_minimal_footprint.conf
@@ -25,7 +25,6 @@
 CONFIG_MAIN_STACK_SIZE=10240
 CONFIG_THREAD_STACK_INFO=n
 # CONFIG_TICKLESS_KERNEL is not set
-CONFIG_SYSTEM_CLOCK_DISABLE=y
 CONFIG_FLASH=y
 
 CONFIG_CONSOLE=n
diff --git a/boot/zephyr/boards/thingy53_nrf5340_cpuapp.conf b/boot/zephyr/boards/thingy53_nrf5340_cpuapp.conf
index 5d7efdd..8e29a8b 100644
--- a/boot/zephyr/boards/thingy53_nrf5340_cpuapp.conf
+++ b/boot/zephyr/boards/thingy53_nrf5340_cpuapp.conf
@@ -1,6 +1,5 @@
 CONFIG_SIZE_OPTIMIZATIONS=y
 
-CONFIG_SYSTEM_CLOCK_DISABLE=y
 CONFIG_SYSTEM_CLOCK_NO_WAIT=y
 CONFIG_PM=n
 
diff --git a/boot/zephyr/prj.conf b/boot/zephyr/prj.conf
index e18f090..e4c0129 100644
--- a/boot/zephyr/prj.conf
+++ b/boot/zephyr/prj.conf
@@ -1,5 +1,4 @@
 CONFIG_DEBUG=y
-CONFIG_SYSTEM_CLOCK_DISABLE=y
 CONFIG_PM=n
 
 CONFIG_MAIN_STACK_SIZE=10240