Clean up nested scope in GetReservedDestination

Suggested https://github.com/bitcoin/bitcoin/pull/17304#discussion_r341194391
by Gregory Sanders <gsanders87@gmail.com>

Reason for keeping the `return true` `return false` verbosity is that more code
will be added after the ReserveKeyFromKeyPool() call before returning.

src/wallet/scriptpubkeyman.cpp

@@ -264,10 +264,8 @@ bool LegacyScriptPubKeyMan::EncryptKeys(CKeyingMaterial& vMasterKeyIn)
 
 bool LegacyScriptPubKeyMan::GetReservedDestination(const OutputType type, bool internal, int64_t& index, CKeyPool& keypool)
 {
-    {
-        if (!ReserveKeyFromKeyPool(index, keypool, internal)) {
-            return false;
-        }
+    if (!ReserveKeyFromKeyPool(index, keypool, internal)) {
+        return false;
     }
     return true;
 }