Merge bitcoin/bitcoin#26965: refactor: Remove stray cs_main redundant declaration

faba08b5b4 refactor: Remove stray cs_main redundant declaration (MarcoFalke)
fa02591edf doc: Export threadsafety.h from sync.h (MarcoFalke)

Pull request description:

  Looks like this was forgotten when introducing kernel/cs_main ?

  Also, there is a commit to export threadsafety.h from sync.h.

ACKs for top commit:
  hebasto:
    ACK faba08b5b4

Tree-SHA512: 0aa58e7693b6fcd504f9da7339f8baa463a6407f67b27f68002db705f4642321ac3765f16c3d906c925ee24085591b79160a62fa5f4aaf6f2e5dcc788411800d

src/node/utxo_snapshot.cpp

@@ -7,12 +7,17 @@
 #include <fs.h>
 #include <logging.h>
 #include <streams.h>
+#include <sync.h>
+#include <tinyformat.h>
+#include <txdb.h>
 #include <uint256.h>
 #include <util/system.h>
 #include <validation.h>
 
+#include <cassert>
 #include <cstdio>
 #include <optional>
+#include <string>
 
 namespace node {
 

src/node/utxo_snapshot.h

@@ -7,13 +7,16 @@
 #define BITCOIN_NODE_UTXO_SNAPSHOT_H
 
 #include <fs.h>
-#include <uint256.h>
+#include <kernel/cs_main.h>
 #include <serialize.h>
-#include <validation.h>
+#include <sync.h>
+#include <uint256.h>
 
+#include <cstdint>
 #include <optional>
+#include <string_view>
 
-extern RecursiveMutex cs_main;
+class Chainstate;
 
 namespace node {
 //! Metadata describing a serialized version of a UTXO set from which an