samples/littlefs: Remove FIXME with PRIxPTR usage

With "Incompatible (u)intptr_t type and PRIxPTR definitions",
https://github.com/zephyrproject-rtos/zephyr/issues/37718

,
issue resolved the FIXME code is no longer needed.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>