Merge bitcoin/bitcoin#22609: [GetTransaction] remove unneeded cs_main lock acquire

4a1b2a7 [GetTransaction] remove unneeded `cs_main` lock acquire (Sebastian Falbesoner)

Pull request description:

  This PR is a follow-up to #22383. For reading from the mempool, only `mempool.cs` needs to be locked (see [suggestion by MarcoFalke](bitcoin/bitcoin#22383 (comment))):

  https://github.com/bitcoin/bitcoin/blob/b620b2d58a55a88ad21da70cb2000863ef17b651/src/txmempool.h#L554-L558

  `CTxMemPool::get()` acquires this lock:

  https://github.com/bitcoin/bitcoin/blob/b620b2d58a55a88ad21da70cb2000863ef17b651/src/txmempool.cpp#L822-L829

   so we don't need to acquire any lock ourselves in `GetTransaction()`, as the other functions called in the remaining parts also don't need to have `cs_main` locked.

ACKs for top commit:
  tryphe:
    Concept ACK. tested 4a1b2a7 but not extensively.
  jnewbery:
    Code review ACK 4a1b2a7

Tree-SHA512: 60e869f72e65cf72cb144be1900ea7f3d87c12f322756994f6a3ed8cd975230b36c7c90c34b60bbf41f9186f4add36decaac1d4f0d0749fb5451b3938a8aa78c

Author: fanquake

src/node/transaction.cpp

@@ -125,8 +125,6 @@ TransactionError BroadcastTransaction(NodeContext& node, const CTransactionRef t
 
 CTransactionRef GetTransaction(const CBlockIndex* const block_index, const CTxMemPool* const mempool, const uint256& hash, const Consensus::Params& consensusParams, uint256& hashBlock)
 {
-    LOCK(cs_main);
-
     if (mempool && !block_index) {
         CTransactionRef ptx = mempool->get(hash);
         if (ptx) return ptx;