rpc: Remove submitblock coinbase pre-check

The coinbase check is repeated again early during ProcessNewBlock.
Pre-checking it may also shadow more fundamental problems with a block.
In most cases the block header is checked first, before validating the
transactions. Checking the coinbase first therefore masks potential
issues with the header. Fix this by removing the pre-check.

The pre-check was likely introduced on top of
ada0caa165 to fix UB in
GetWitnessCommitmentIndex in case a block's transactions are empty. This
code path could only be reached because of the call to
UpdateUncommittedBlockStructures in submitblock, but cannot be reached
through net_processing.

Add some functional test cases to cover the previous conditions that
lead to a "Block does not start with a coinbase" json rpc error being
returned.

---

With the introduction of a mining ipc interface and the potential future
introduction of a kernel library API it becomes increasingly important
to offer common behaviour between them. An example of this is
ProcessNewBlock, which is used by ipc, rpc, net_processing and
(potentially) the kernel library. Having divergent behaviour on
suggested pre-checks and checks for these functions is confusing to both
developers and users and is a maintenance burden.

The rpc interface for ProcessNewBlock (submitblock) currently pre-checks
if the block has a coinbase transaction and whether it has been
processed before. While the current example binary for how to use the
kernel library, bitcoin-chainstate, imitates these checks, the other
interfaces do not.

src/rpc/mining.cpp

@@ -1015,10 +1015,6 @@ static RPCHelpMan submitblock()
         throw JSONRPCError(RPC_DESERIALIZATION_ERROR, "Block decode failed");
     }
 
-    if (block.vtx.empty() || !block.vtx[0]->IsCoinBase()) {
-        throw JSONRPCError(RPC_DESERIALIZATION_ERROR, "Block does not start with a coinbase");
-    }
-
     ChainstateManager& chainman = EnsureAnyChainman(request.context);
     uint256 hash = block.GetHash();
     {