pico-imx7d: README: Drop old instructions about secure mode

Our default config already has the secure mode supported, so the
manual step is not required anymore.
Signed-off-by: NOtavio Salvador <otavio@ossystems.com.br>

board/technexion/pico-imx7d/README

@@ -52,20 +52,3 @@ Remove power from the pico board.
 Put pico board into normal boot mode.
 
 Power up the board and the new updated U-Boot should boot from eMMC.
-
-Building U-Boot to boot with NXP 4.1 kernel:
-
-The NXP 4.1 kernel boots only in secure boot mode on mx7.
-
-Follow the next steps to enable secure boot:
-
-$ make mrproper
-$ make pico-imx7d_defconfig
-$ make menuconfig
-	-> ARM architecture
-	-> [*] Enable support for booting in non-secure mode
-	-> [*]   Boot in secure mode by default
-	-> Exit
-$ make
-
-Flash u-boot.imx using the imx_usb_loader tool.