refactor(mbedtls): remove hack in LIBMBEDTLS_CFLAGS

MbedTLS 3.6.1 fixed the issue which previously
produced this warning, so this hack is no longer
necessary.

Change-Id: I934adefbf2fed16e16b9d98bc8674125b70b08fc
Signed-off-by: Ryan Everett <ryan.everett@arm.com>
diff --git a/drivers/auth/mbedtls/mbedtls_common.mk b/drivers/auth/mbedtls/mbedtls_common.mk
index e925e14..998c491 100644
--- a/drivers/auth/mbedtls/mbedtls_common.mk
+++ b/drivers/auth/mbedtls/mbedtls_common.mk
@@ -66,7 +66,6 @@
 					)
 
 ifeq (${PSA_CRYPTO},1)
-LIBMBEDTLS_CFLAGS 	+= -Wno-error=unused-but-set-variable
 LIBMBEDTLS_SRCS         += $(addprefix ${MBEDTLS_DIR}/library/,    	\
 					psa_crypto.c                   	\
 					psa_crypto_client.c            	\