[validation] Remove comment about AcceptToMemoryPool()

"This logic is not necessary for memory pool transactions, as
AcceptToMemoryPool already refuses previously-known transaction ids
entirely." refers to the logic at
https://github.com/bitcoin/bitcoin/blob/a206b0ea12eb4606b93323268fc81a4f1f952531/src/main.cpp#L484-L486,
which was later removed in commit 450cbb0.

Author: jnewbery

src/validation.cpp

@@ -1658,8 +1658,6 @@ bool CChainState::ConnectBlock(const CBlock& block, BlockValidationState& state,
     // can be duplicated to remove the ability to spend the first instance -- even after
     // being sent to another address.
     // See BIP30, CVE-2012-1909, and http://r6.ca/blog/20120206T005236Z.html for more information.
-    // This logic is not necessary for memory pool transactions, as AcceptToMemoryPool
-    // already refuses previously-known transaction ids entirely.
     // This rule was originally applied to all blocks with a timestamp after March 15, 2012, 0:00 UTC.
     // Now that the whole chain is irreversibly beyond that time it is applied to all blocks except the
     // two in the chain that violate it. This prevents exploiting the issue against nodes during their