[libc++] Remove unneeded visibility pragmas

[smeenai]

Summary

The function definitions being guarded by the pragma were all static, so
they wouldn't be exported anyway. In any case, we should prefer the
visibility macros. No functional change.

src/chrono.cpp

 //   mach_absolute_time() * MachInfo.numer / MachInfo.denom is the number of
 //   nanoseconds since the computer booted up.  MachInfo.numer and MachInfo.denom
 //   are run time constants supplied by the OS.  This clock has no relationship
 //   to the Gregorian calendar.  It's main use is as a high resolution timer.
 
 // MachInfo.numer / MachInfo.denom is often 1 on the latest equipment.  Specialize
 //   for that case as an optimization.
 
-#pragma GCC visibility push(hidden)
-
 static
 steady_clock::rep
 steady_simplified()
 {
     return static_cast<steady_clock::rep>(mach_absolute_time());
 }
 
 static

 {
     mach_timebase_info_data_t MachInfo;
     mach_timebase_info(&MachInfo);
     if (MachInfo.numer == MachInfo.denom)
         return &steady_simplified;
     return &steady_full;
 }
 
-#pragma GCC visibility pop
-
 steady_clock::time_point
 steady_clock::now() _NOEXCEPT
 {
     static FP fp = init_steady_clock();
     return time_point(duration(fp()));
 }
 
 #else
 #error "Monotonic clock not implemented"
 #endif
 
 #endif // !_LIBCPP_HAS_NO_MONOTONIC_CLOCK
 
 }
 
 _LIBCPP_END_NAMESPACE_STD