fs: littlefs: sample: Move littlefs_flash_erase() to flash mount
fs: littlefs: sample: Move littlefs_flash_erase() to flash mount

The littlefs_flash_erase() function shall be called from flash specific
littlefs_mount function. This allows removal of the stub function
when block device support is added.

Signed-off-by: Lukasz Majewski <7acbb0643e3da1a0f6f894ebd720fb3807e67dff@denx.de>

finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr