boards/qemu_x86: fix emulated program memory size

Qemu_x86 didn't reflect emulated program memory size.
It was because chosen zephyr,flash was assigned to flash_simulator
which was helping to generate DT_FLASH properties for sim_flash node.

This change revert choice of flash0 which solve problem with
program memory size. Flash simulator have to use
DT_SOC_NV_FLASH_xxx labels for fetch its property since that.

fixes #15832

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>