Merge bitcoin/bitcoin#19909: refactor: Remove unused CTxMemPool::clear() helper

fa818e103c txmempool: Remove unused clear() member function (MarcoFalke)

Pull request description:

  Seems odd to have code in Bitcoin Core that is unused.

  Moreover the function was broken (see https://github.com/bitcoin/bitcoin/pull/24145) and is brittle, as there is nothing that prevents similar bugs from re-appearing.

  Fix both issues by replacing it with C++11 member initializers.

ACKs for top commit:
  glozow:
    ACK fa818e103c

Tree-SHA512: e79e44cac7d5a84d9ecc8e3f3b0b9a50e1e3ebec358b20ba5dac175ef07d1fbe338a20f83ee80f746f7c726c79e77f8be49e14bca57a41063da8a5302123c3a9

src/txmempool.cpp

@@ -396,7 +396,6 @@ CTxMemPool::CTxMemPool(const Options& opts)
       m_full_rbf{opts.full_rbf},
       m_limits{opts.limits}
 {
-    _clear(); //lock free clear
 }
 
 bool CTxMemPool::isSpent(const COutPoint& outpoint) const
@@ -634,26 +633,6 @@ void CTxMemPool::removeForBlock(const std::vector<CTransactionRef>& vtx, unsigne
     blockSinceLastRollingFeeBump = true;
 }
 
-void CTxMemPool::_clear()
-{
-    vTxHashes.clear();
-    mapTx.clear();
-    mapNextTx.clear();
-    totalTxSize = 0;
-    m_total_fee = 0;
-    cachedInnerUsage = 0;
-    lastRollingFeeUpdate = GetTime();
-    blockSinceLastRollingFeeBump = false;
-    rollingMinimumFeeRate = 0;
-    ++nTransactionsUpdated;
-}
-
-void CTxMemPool::clear()
-{
-    LOCK(cs);
-    _clear();
-}
-
 void CTxMemPool::check(const CCoinsViewCache& active_coins_tip, int64_t spendheight) const
 {
     if (m_check_ratio == 0) return;