drivers: flash: native_posix: remove unused mutex

Remove unused mutex in the flash_native_posix driver.

Signed-off-by: Henrik Brix Andersen <hebad@vestas.com>
diff --git a/drivers/flash/flash_native_posix.c b/drivers/flash/flash_native_posix.c
index ea1d698..fb2b2ea 100644
--- a/drivers/flash/flash_native_posix.c
+++ b/drivers/flash/flash_native_posix.c
@@ -25,7 +25,6 @@
 static const char default_flash_path[] = "flash.bin";
 
 struct flash_native_posix_data {
-	struct k_sem mutex;
 	const char *flash_path;
 	int fd;
 	u8_t *flash;
@@ -131,8 +130,6 @@
 
 	data->init_called = true;
 
-	k_sem_init(&data->mutex, 1, 1);
-
 	if (data->flash_path == NULL) {
 		data->flash_path = default_flash_path;
 	}