Merge bitcoin/bitcoin#28890: rpc: Remove deprecated -rpcserialversion

fa46cc22bc Remove deprecated -rpcserialversion (MarcoFalke)

Pull request description:

  The flag is problematic for many reasons:

  * It is deprecated
  * It is a global flag, requiring a restart to change, as opposed to a flag that can be set on each RPC invocation
  * It may be hidden in config files by accident, hard to debug, causing LND crashes and bugs, see https://github.com/bitcoin/bitcoin/issues/28730#issuecomment-1780940868
  * It makes performance improvements harder to implement: https://github.com/bitcoin/bitcoin/pull/17529#issuecomment-556082818

  Fix all issues by removing it.

  If there is a use-case, likely a per-RPC flag can be added, if needed.

ACKs for top commit:
  ajtowns:
    crACK fa46cc22bc
  TheCharlatan:
    lgtm ACK fa46cc22bc

Tree-SHA512: 96ba1c60356ce93954fe5c2a59045771c6d1516ad0d9dc436ef1800a1f1b0153f0d5fb78ca99d53ad54ba25fbce36962bdf1d4325aceedfc8154a61347a6a915

src/rpc/server.cpp

@@ -597,9 +597,4 @@ void RPCRunLater(const std::string& name, std::function<void()> func, int64_t nS
     deadlineTimers.emplace(name, std::unique_ptr<RPCTimerBase>(timerInterface->NewTimer(func, nSeconds*1000)));
 }
 
-bool RPCSerializationWithoutWitness()
-{
-    return (gArgs.GetIntArg("-rpcserialversion", DEFAULT_RPC_SERIALIZE_VERSION) == 0);
-}
-
 CRPCTable tableRPC;