net: openthread: Remove old flash code

Since commit b3a1ede8 OpenThread uses the Zepyhr settings submodule
instead of writing to the flash directly. The flash.c file is not
compiled anymore, so let's just remove it. Also remove the
OT_PLAT_FLASH_PAGES_COUNT Kconfig option which was solely used by that
file.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>