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.cpp

@@ -5007,22 +5007,6 @@ bool ChainstateManager::IsSnapshotActive() const
     return m_snapshot_chainstate && m_active_chainstate == m_snapshot_chainstate.get();
 }
 
-CChainState& ChainstateManager::ValidatedChainstate() const
-{
-    LOCK(::cs_main);
-    if (m_snapshot_chainstate && IsSnapshotValidated()) {
-        return *m_snapshot_chainstate.get();
-    }
-    assert(m_ibd_chainstate);
-    return *m_ibd_chainstate.get();
-}
-
-bool ChainstateManager::IsBackgroundIBD(CChainState* chainstate) const
-{
-    LOCK(::cs_main);
-    return (m_snapshot_chainstate && chainstate == m_ibd_chainstate.get());
-}
-
 void ChainstateManager::Unload()
 {
     for (CChainState* chainstate : this->GetAll()) {