tests: boot: mcuboot_recovery_retention: Remove boot mode overlay

Removes setting the boot mode device in the default test overlay
file as this is now set by the board

Signed-off-by: Jamie McCrae <[email protected]>

Author: nordicjm

tests/boot/mcuboot_recovery_retention/boards/nrf52840dk_nrf52840.overlay

@@ -2,21 +2,10 @@
 
 / {
 	chosen {
-		zephyr,boot-mode = &boot_mode0;
 		zephyr,uart-mcumgr = &cdc_acm_uart;
 	};
 };
 
-&gpregret1 {
-	status = "okay";
-
-	boot_mode0: boot_mode@0 {
-		compatible = "zephyr,retention";
-		status = "okay";
-		reg = <0x0 0x1>;
-	};
-};
-
 &zephyr_udc0 {
 	status = "okay";