Stop using a mutex for guarding CachedCalendar.

src/modules/clock.cpp

@@ -1,5 +1,4 @@
 #include "modules/clock.hpp"
-#include <mutex>
 #include <sstream>
 
 using zoned_time = date::zoned_time<std::chrono::system_clock::duration>;
@@ -37,14 +36,11 @@ struct CachedCalendar {
   }
 };
 
-std::mutex cached_calendar_mutex;  // protects cached_calendar.
 CachedCalendar cached_calendar;
 
 std::string calendar_text(const waybar_time& wtime, const date::weekday& first_dow) {
   const auto daypoint = date::floor<date::days>(wtime.ztime.get_local_time());
   const auto ymd = date::year_month_day(daypoint);
-
-  const std::lock_guard<std::mutex> lock(cached_calendar_mutex);
   if (cached_calendar.ymd == ymd) {
     return cached_calendar.text;
   }