clang: fix some compiler warnings

From CI.

lib/mu-server.cc

@@ -119,7 +119,6 @@ private:
 	std::string		fname_;
 	using OutType = std::variant<std::ofstream, std::ostringstream>;
 	OutType out_;
-	bool do_unlink_{};
 };
 
 

lib/mu-store.cc

@@ -655,7 +655,7 @@ Store::maildirs() const
 
 	Scanner::Handler handler = [&](const std::string& path, auto&& _1, auto&& _2) {
 		auto md{path.substr(prefix_size)};
-		mdirs.emplace_back(std::move(md.empty() ? "/" : md));
+		mdirs.emplace_back(md.empty() ? "/" : std::move(md));
 		return true;
 	};
 

lib/mu-xapian-db.hh

@@ -536,10 +536,7 @@ private:
 
 	mutable std::mutex lock_;
 	std::string path_;
-
 	DbType	db_;
-	bool	in_tx_;
-
 	size_t	tx_level_{};
 	const size_t batch_size_;
 	size_t changes_{};

lib/utils/mu-logger.cc

@@ -209,7 +209,7 @@ test_logger_threads(void)
 	/* log to the logger file from many threass */
 	for (auto n = 0; n != thread_num; ++n)
 		threads.emplace_back(
-			std::thread([n,&running]{
+			std::thread([&running]{
 				while (running) {
 					//mu_debug("log message from thread <{}>", n);
 					std::this_thread::yield();