keystore GetKeys(): return result instead of writing to reference

Issue: #10905

By returning the result, a few useless lines can be removed.

Return-value-optimization means there should be no copy.

src/wallet/wallet.cpp

@@ -3660,13 +3660,10 @@ void CWallet::GetKeyBirthTimes(std::map<CTxDestination, int64_t> &mapKeyBirth) c
     // map in which we'll infer heights of other keys
     CBlockIndex *pindexMax = chainActive[std::max(0, chainActive.Height() - 144)]; // the tip can be reorganized; use a 144-block safety margin
     std::map<CKeyID, CBlockIndex*> mapKeyFirstBlock;
-    std::set<CKeyID> setKeys;
-    GetKeys(setKeys);
-    for (const CKeyID &keyid : setKeys) {
+    for (const CKeyID &keyid : GetKeys()) {
         if (mapKeyBirth.count(keyid) == 0)
             mapKeyFirstBlock[keyid] = pindexMax;
     }
-    setKeys.clear();
 
     // if there are no such keys, we're done
     if (mapKeyFirstBlock.empty())