Refactor: omit parens in lambdas with no parameters

TIL []{} is a thing.  We already do that in some places, so this improves
consistency, although it may be less obvious.

src/common.cpp

@@ -151,7 +151,7 @@ bool is_windows_subsystem_for_linux() {
     // routine simultaneously the first time around, we just end up needlessly querying uname(2) one
     // more time.
 
-    static bool wsl_state = []() {
+    static bool wsl_state = [] {
         utsname info;
         uname(&info);
 
@@ -2130,7 +2130,7 @@ std::string get_path_to_tmp_dir() {
 // session. We err on the side of assuming it's not a console session. This approach isn't
 // bullet-proof and that's OK.
 bool is_console_session() {
-    static const bool console_session = []() {
+    static const bool console_session = [] {
         ASSERT_IS_MAIN_THREAD();
 
         const char *tty_name = ttyname(0);

src/parser_keywords.cpp

@@ -45,7 +45,7 @@ bool parser_keywords_skip_arguments(const wcstring &cmd) {
 }
 
 bool parser_keywords_is_subcommand(const wcstring &cmd) {
-    const static string_set_t search_list = ([]() {
+    const static string_set_t search_list = ([] {
         string_set_t results;
         results.insert(std::begin(subcommand_keywords), std::end(subcommand_keywords));
         results.insert(std::begin(skip_keywords), std::end(skip_keywords));
@@ -68,7 +68,7 @@ bool parser_keywords_is_block(const wcstring &word) {
 }
 
 bool parser_keywords_is_reserved(const wcstring &word) {
-    const static string_set_t search_list = ([]() {
+    const static string_set_t search_list = ([] {
         string_set_t results;
         results.insert(std::begin(subcommand_keywords), std::end(subcommand_keywords));
         results.insert(std::begin(skip_keywords), std::end(skip_keywords));

src/timer.cpp

@@ -212,7 +212,7 @@ static void pop_timer() {
 }
 
 cleanup_t push_timer(bool enabled) {
-    if (!enabled) return {[]() {}};
+    if (!enabled) return {[] {}};
     active_timers.emplace_back(timer_snapshot_t::take());
-    return {[]() { pop_timer(); }};
+    return {[] { pop_timer(); }};
 }