core: remove any tx pubkey from extra before adding one

This will happen when signing a transaction from a cold wallet,
and we don't want the placeholder the hot wallet put in it.

src/cryptonote_core/cryptonote_format_utils.cpp

@@ -480,6 +480,7 @@ namespace cryptonote
 
     tx.extra = extra;
     keypair txkey = keypair::generate();
+    remove_field_from_tx_extra(tx.extra, typeid(tx_extra_pub_key));
     add_tx_pub_key_to_extra(tx, txkey.pub);
     tx_key = txkey.sec;