assumeutxo: remove snapshot during -reindex{-chainstate}

Removing a snapshot chainstate from disk (and memory) is consistent with
existing reindex operations.

src/validation.h

@@ -848,9 +848,6 @@ private:
     //! Points to either the ibd or snapshot chainstate; indicates our
     //! most-work chain.
     //!
-    //! Once this pointer is set to a corresponding chainstate, it will not
-    //! be reset until init.cpp:Shutdown().
-    //!
     //! This is especially important when, e.g., calling ActivateBestChain()
     //! on all chainstates because we are not able to hold ::cs_main going into
     //! that call.
@@ -1203,6 +1200,10 @@ public:
 
     void ResetChainstates() EXCLUSIVE_LOCKS_REQUIRED(::cs_main);
 
+    //! Remove the snapshot-based chainstate and all on-disk artifacts.
+    //! Used when reindex{-chainstate} is called during snapshot use.
+    [[nodiscard]] bool DeleteSnapshotChainstate() EXCLUSIVE_LOCKS_REQUIRED(::cs_main);
+
     //! Switch the active chainstate to one based on a UTXO snapshot that was loaded
     //! previously.
     Chainstate& ActivateExistingSnapshot(CTxMemPool* mempool, uint256 base_blockhash)