Merge bitcoin/bitcoin#22626: Remove txindex migration code

fa20f815a9 Remove txindex migration code (MarcoFalke)
fae8786033 doc: Fix validation typo (MarcoFalke)
fab89006d6 Add missing includes and forward declarations, remove unused ones (MarcoFalke)

Pull request description:

  No supported version of Bitcoin Core used the legacy txindex, so all relevant nodes can be assumed to have upgraded. Thus, there is no need to keep this code any longer.

  As a temporary courtesy, provide a one-time warning on how to free the disk space used by the legacy txindex.

  Fixes #22615

ACKs for top commit:
  laanwj:
    Code review ACK fa20f815a9
  hebasto:
    ACK fa20f815a9, tested on Linux Mint 20.2 (x86_64).
  Zero-1729:
    crACK fa20f815a9
  theStack:
    Approach ACK fa20f815a9

Tree-SHA512: 68aa32d064d1e3932e6e382816a4b5de417bd7e82861fea1ee50660e8c397f4efeb88ae4ed54a8ad1952c3563eb0b8449d7ccf883c353cc4d4dc7e15c53d78e8

src/validation.h

@@ -11,7 +11,9 @@
 #endif
 
 #include <amount.h>
+#include <arith_uint256.h>
 #include <attributes.h>
+#include <chain.h>
 #include <coins.h>
 #include <consensus/validation.h>
 #include <crypto/common.h> // for ReadLE64
@@ -21,10 +23,11 @@
 #include <policy/packages.h>
 #include <protocol.h> // For CMessageHeader::MessageStartChars
 #include <script/script_error.h>
-#include <sync.h>
-#include <txmempool.h> // For CTxMemPool::cs
-#include <txdb.h>
 #include <serialize.h>
+#include <sync.h>
+#include <txdb.h>
+#include <txmempool.h> // For CTxMemPool::cs
+#include <uint256.h>
 #include <util/check.h>
 #include <util/hasher.h>
 #include <util/translation.h>
@@ -42,7 +45,6 @@
 
 class CChainState;
 class BlockValidationState;
-class CBlockIndex;
 class CBlockTreeDB;
 class CBlockUndo;
 class CChainParams;