Remove unnecessary call to mbedtls_mpi_free() in MPI tests

tests/suites/test_suite_mpi.function

@@ -257,8 +257,6 @@ void mpi_invalid_param( )
     TEST_INVALID_PARAM_RET( MBEDTLS_ERR_MPI_BAD_INPUT_DATA,
                             mbedtls_mpi_inv_mod( &X, &X, NULL ) );
 
-    mbedtls_mpi_free( NULL );
-
 exit:
     return;
 }