boot: zephyr: boards: stm32h750b_dk_ext_flash_app: remove conf file

STM32_MEMMAP is no longer needed since it is now set on the Zephyr side
in the board's 'Kconfig.defconfig' when building MCUboot app.

Also, BOOT_MAX_IMG_SECTORS_AUTO no longer need to be disabled after
including the 'soc-nv-flash' binding in the 'st,stm32-qspi-nor' binding,
then adding the 'erase-block-size' & 'write-block-size' properties
in external Flash DT node.

Signed-off-by: Abderrahmane JARMOUNI <[email protected]>

Author: JarmouniA