chore: not more required blockchain beeing fully synced to refuse block conflicting to chainlocks

Check if blockchain synced is already done ChainLockSigner::TrySignChainTip but in other places it does not seem useful

Author: knst

src/instantsend/instantsend.cpp

@@ -476,9 +476,6 @@ bool CInstantSendManager::IsInstantSendEnabled() const
 
 bool CInstantSendManager::RejectConflictingBlocks() const
 {
-    if (!m_mn_sync.IsBlockchainSynced()) {
-        return false;
-    }
     if (!spork_manager.IsSporkActive(SPORK_3_INSTANTSEND_BLOCK_FILTERING)) {
         LogPrint(BCLog::INSTANTSEND, "%s: spork3 is off, skipping transaction locking checks\n", __func__);
         return false;