Unit tests: Remove `#ifndef UNITTEST` from the mbed_assert.h stub

The stub version of mbed_assert.h is never used outside unit tests,
so the check on `UNITTEST` is redundant.

UNITTESTS/target_h/platform/mbed_assert.h

@@ -41,9 +41,7 @@ extern "C" {
  */
 // mbed_assert_internal UT stub only prints an assert trace and returns, so therefore
 // MBED_NORETURN must not be defined for UTs.
-#ifndef UNITTEST
 MBED_NORETURN
-#endif
 void mbed_assert_internal(const char *expr, const char *file, int line);
 
 #ifdef __cplusplus