[wallet] Remove return value from CommitTransaction()

CommitTransaction returns a bool to indicate success, but since commit
b3a74100b8 it only returns true, even if the transaction was not
successfully broadcast. This commit changes CommitTransaction() to return
void.

All dead code in `if (!CommitTransaction())` branches has been removed.

src/wallet/test/wallet_tests.cpp

@@ -452,7 +452,7 @@ public:
             BOOST_CHECK(wallet->CreateTransaction(*locked_chain, {recipient}, tx, fee, changePos, error, dummy));
         }
         CValidationState state;
-        BOOST_CHECK(wallet->CommitTransaction(tx, {}, {}, state));
+        wallet->CommitTransaction(tx, {}, {}, state);
         CMutableTransaction blocktx;
         {
             LOCK(wallet->cs_wallet);