Remove NULL-ing of passed in SSL context in ssl_populate_transform()

Remove a piece of code that was meant to ensure non-usage of the ssl
context (by NULL-ing it) under conditions where it should not be used,
as this now makes less sense.

Signed-off-by: Paul Elliott <paul.elliott@arm.com>

library/ssl_tls.c

@@ -1004,8 +1004,7 @@ static int ssl_populate_transform(mbedtls_ssl_transform *transform,
     !defined(MBEDTLS_SSL_EXPORT_KEYS) && \
     !defined(MBEDTLS_SSL_DTLS_CONNECTION_ID) && \
     !defined(MBEDTLS_DEBUG_C)
-    ssl = NULL; /* make sure we don't use it except for those cases */
-    (void) ssl;
+    (void) ssl; /* ssl is unused except for those cases */
 #endif
 
     /*