refactor: CTxMemPool::GetTotalTxSize() requires CTxMemPool::cs lock

No change in behavior, the lock is already held at call sites.

src/txmempool.h

@@ -710,9 +710,9 @@ public:
         return mapTx.size();
     }
 
-    uint64_t GetTotalTxSize() const
+    uint64_t GetTotalTxSize() const EXCLUSIVE_LOCKS_REQUIRED(cs)
     {
-        LOCK(cs);
+        AssertLockHeld(cs);
         return totalTxSize;
     }