[nrf noup] Fix duplicate declaration of ad_len_field

squash! [nrf noup] Backport DTLS CID support compilant with RFC-9146

MBEDTLS_SSL_DTLS_CONNECTION_ID fails to build now because ad_len_field
is defined twice. Remove the extra one.

Regression from: cf39070
Should be squashed with that commit in next upmerge or rebase.

Signed-off-by: Pete Skeggs <peter.skeggs@nordicsemi.no>
(cherry picked from commit 30c4a6c)

Author: plskeggs

library/ssl_msg.c

@@ -521,7 +521,6 @@ static void ssl_extract_add_data_from_record( unsigned char* add_data,
     size_t ad_len_field = rec->data_len;
 
 #if defined(MBEDTLS_SSL_DTLS_CONNECTION_ID)
-    size_t ad_len_field = rec->data_len;
     const unsigned char seq_num_placeholder[] = { 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff };
 #endif /* MBEDTLS_SSL_DTLS_CONNECTION_ID */