tests/drivers/flash_simulator: Remove native_posix overlays

native_posix is being removed as it has reached its end of life in 4.2,
after being deprecated since 4.0.

So these overlays will not server any purpose anymore.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>

tests/drivers/flash_simulator/flash_sim_impl/boards/native_posix.overlay

@@ -1,7 +0,0 @@
-/*
- * Copyright (c) 2023 Nordic Semiconductor ASA
- *
- * SPDX-License-Identifier: Apache-2.0
- */
-
-#include "native_sim.overlay"

tests/drivers/flash_simulator/flash_sim_impl/boards/native_posix_native_64.overlay

@@ -1,7 +0,0 @@
-/*
- * Copyright (c) 2024 Nordic Semiconductor ASA
- *
- * SPDX-License-Identifier: Apache-2.0
- */
-
-#include "native_posix.overlay"