Threat Model: adjust to 2.28

MBEDTLS_AESCE_C is not available in 2.28., remove it from workarounds.

Signed-off-by: Janos Follath <janos.follath@arm.com>
diff --git a/SECURITY.md b/SECURITY.md
index 8d3678a..e25601b 100644
--- a/SECURITY.md
+++ b/SECURITY.md
@@ -129,7 +129,7 @@
 
 - Turn on hardware acceleration for AES. This is supported only on selected
   architectures and currently only available for AES. See configuration options
-  `MBEDTLS_AESCE_C`, `MBEDTLS_AESNI_C` and `MBEDTLS_PADLOCK_C` for details.
+  `MBEDTLS_AESNI_C` and `MBEDTLS_PADLOCK_C` for details.
 - Add a secure alternative implementation (typically hardware acceleration) for
   the vulnerable cipher. See the [Alternative Implementations
 Guide](docs/architecture/alternative-implementations.md) for more information.