wallet: batch and simplify ZapSelectTx process

The goal of the function is to erase the wallet transactions that
match the inputted hashes. There is no need to traverse the database,
reading record by record, to then perform single entry removals for
each of them.

To ensure consistency and improve performance, this change-set removes
all tx records within a single atomic db batch operation, as well as
it cleans up code, improves error handling and simplifies the
transactions removal process entirely.

This optimizes the removal of watch-only transactions during the wallet
migration process and the 'removeprunedfunds' RPC command.

src/wallet/test/wallet_tests.cpp

@@ -918,8 +918,8 @@ BOOST_FIXTURE_TEST_CASE(ZapSelectTx, TestChain100Setup)
         BOOST_CHECK(wallet->HasWalletSpend(prev_tx));
         BOOST_CHECK_EQUAL(wallet->mapWallet.count(block_hash), 1u);
 
-        std::vector<uint256> vHashIn{ block_hash }, vHashOut;
-        BOOST_CHECK_EQUAL(wallet->ZapSelectTx(vHashIn, vHashOut), DBErrors::LOAD_OK);
+        std::vector<uint256> vHashIn{ block_hash };
+        BOOST_CHECK(wallet->ZapSelectTx(vHashIn));
 
         BOOST_CHECK(!wallet->HasWalletSpend(prev_tx));
         BOOST_CHECK_EQUAL(wallet->mapWallet.count(block_hash), 0u);