Unit tests: Do not include mbed.h in storage stubs

The header `mbed.h` pulls in headers from a number of Mbed OS
components that are not necessarily used. It was intended for user
applications only, and libraries and unit tests should explicitly
include library headers it uses to limit dependencies.

This change avoids having to link unnecessary libraries in storage
unit tests' CMake definitions.

Author: LDong-Arm

storage/blockdevice/tests/UNITTESTS/doubles/ExhaustibleBlockDevice_stub.cpp

@@ -16,7 +16,6 @@
  */
 
 #include "ExhaustibleBlockDevice.h"
-#include "mbed.h"
 #include "mbed_critical.h"
 
 

storage/blockdevice/tests/UNITTESTS/doubles/ObservingBlockDevice_stub.cpp

@@ -17,7 +17,6 @@
 
 #include "ObservingBlockDevice.h"
 #include "ReadOnlyBlockDevice.h"
-#include "mbed.h"
 
 
 ObservingBlockDevice::ObservingBlockDevice(BlockDevice *bd)