Remove explicit include of flash_data.h in flash api test

The header is already included inside of flash_api.h if the target
uses CMSIS flash algos. If it doesn't, the file shouldn't be included,
since it doesn't exist.

Author: asmellby

TESTS/mbed_hal/flash/functional_tests/main.cpp

@@ -24,7 +24,6 @@
 
 #include "mbed.h"
 #include "flash_api.h"
-#include "flash_data.h"
 
 using namespace utest::v1;