mining: drop unused include_dummy_extranonce option

The previous commit made CreateNewBlock() ignore the
include_dummy_extranonce flag (OP_0 is now appended only at heights
<= 16 to satisfy bad-cb-length, regardless of caller-supplied options).

Remove the now-unused field from BlockCreateOptions and clean up all
the call sites that still set it (RPC, bench, fuzz, test setups).

src/test/fuzz/process_message.cpp

@@ -43,7 +43,6 @@ void ResetChainman(TestingSetup& setup)
     setup.LoadVerifyActivateChainstate();
     for (int i = 0; i < 2 * COINBASE_MATURITY; i++) {
         node::BlockAssembler::Options options;
-        options.include_dummy_extranonce = true;
         MineBlock(setup.m_node, options);
     }
 }