CLEANUP: quic: remove unused function prototype

Removed hexdump unusued prototype from quic_tls.c.

This should be backported up to 2.6.

src/quic_tls.c

@@ -16,9 +16,6 @@ DECLARE_POOL(pool_head_quic_tls_secret, "quic_tls_secret", QUIC_TLS_SECRET_LEN);
 DECLARE_POOL(pool_head_quic_tls_iv,     "quic_tls_iv",     QUIC_TLS_IV_LEN);
 DECLARE_POOL(pool_head_quic_tls_key,    "quic_tls_key",    QUIC_TLS_KEY_LEN);
 
-__attribute__((format (printf, 3, 4)))
-void hexdump(const void *buf, size_t buflen, const char *title_fmt, ...);
-
 /* Initial salt depending on QUIC version to derive client/server initial secrets.
  * This one is for draft-29 QUIC version.
  */