ecp_curves: Update `mbedtls_ecp_group_free()`.

This patch updates the method to not free the `grp->P`
and `grp->N` structure members.

The contents of `P` and `N` are stored in static memory at
`curve448_p/n` and `curve25519p/n` and no longer dynamically
allocated.

Signed-off-by: Minos Galanakis <minos.galanakis@arm.com>
diff --git a/library/ecp.c b/library/ecp.c
index d9d5425..08fbe86 100644
--- a/library/ecp.c
+++ b/library/ecp.c
@@ -582,11 +582,9 @@
     }
 
     if (grp->h != 1) {
-        mbedtls_mpi_free(&grp->P);
         mbedtls_mpi_free(&grp->A);
         mbedtls_mpi_free(&grp->B);
         mbedtls_ecp_point_free(&grp->G);
-        mbedtls_mpi_free(&grp->N);
     }
 
     if (!ecp_group_is_static_comb_table(grp) && grp->T != NULL) {