doc: update BOOTLOADER_MCUBOOT description

mcuboot.overlay is not used any more when BOOTLOADER_MCUBOOT Kconfig
option is selected. Update the documentation accordingly.

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

Kconfig.zephyr

@@ -389,8 +389,6 @@ config BOOTLOADER_MCUBOOT
 	      for the MCUboot image header
 	    * Activating SW_VECTOR_RELAY on Cortex-M0 (or Armv8-M baseline)
 	      targets with no built-in vector relocation mechanisms
-	    * Including dts/common/mcuboot.overlay when building the Device
-	      Tree in order to place and link the image at the slot0 offset
 
 config BOOTLOADER_ESP_IDF
 	bool "ESP-IDF bootloader support"