refactor: remove unused assumeutxo methods

After feedback from Russ, I realized that there are some extraneous assumeutxo methods
that are not necessary and probably just overly confusing. These include

- `Validated*()`
- `IsBackgroundIBD()`

and they can be removed.

src/validation.h

@@ -862,12 +862,6 @@ private:
  * *Background IBD chainstate*: an IBD chainstate for which the
  *    IBD process is happening in the background while use of the
  *    active (snapshot) chainstate allows the rest of the system to function.
- *
- * *Validated chainstate*: the most-work chainstate which has been validated
- *   locally via initial block download. This will be the snapshot chainstate
- *   if a snapshot was loaded and all blocks up to the snapshot starting point
- *   have been downloaded and validated (via background validation), otherwise
- *   it will be the IBD chainstate.
  */
 class ChainstateManager
 {
@@ -986,19 +980,6 @@ public:
     //! Is there a snapshot in use and has it been fully validated?
     bool IsSnapshotValidated() const { return m_snapshot_validated; }
 
-    //! @returns true if this chainstate is being used to validate an active
-    //!          snapshot in the background.
-    bool IsBackgroundIBD(CChainState* chainstate) const;
-
-    //! Return the most-work chainstate that has been fully validated.
-    //!
-    //! During background validation of a snapshot, this is the IBD chain. After
-    //! background validation has completed, this is the snapshot chain.
-    CChainState& ValidatedChainstate() const;
-
-    CChain& ValidatedChain() const { return ValidatedChainstate().m_chain; }
-    CBlockIndex* ValidatedTip() const { return ValidatedChain().Tip(); }
-
     /**
      * Process an incoming block. This only returns after the best known valid
      * block is made active. Note that it does not, however, guarantee that the