Use clock_gettime on Apple platforms (#35923)

Apple platforms have supported clock_gettime for several years now (since 2016).

Simplify implementation and remove required api call to `mach_absolute_time`.

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Closes #35923

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/35923 from dmaclach:patch-4 45f670d10e
PiperOrigin-RevId: 612965724

src/core/lib/gpr/posix/time.cc

@@ -45,7 +45,6 @@ static struct timespec timespec_from_gpr(gpr_timespec gts) {
   return rv;
 }
 
-#if _POSIX_TIMERS > 0 || defined(__OpenBSD__)
 static gpr_timespec gpr_from_timespec(struct timespec ts,
                                       gpr_clock_type clock_type) {
   //
@@ -83,65 +82,6 @@ static gpr_timespec now_impl(gpr_clock_type clock_type) {
     return gpr_from_timespec(now, clock_type);
   }
 }
-#else
-// For some reason Apple's OSes haven't implemented clock_gettime.
-
-#include <mach/mach.h>
-#include <mach/mach_time.h>
-#include <sys/time.h>
-
-static double g_time_scale = []() {
-  mach_timebase_info_data_t tb = {0, 1};
-  mach_timebase_info(&tb);
-  return static_cast<double>(tb.numer) / static_cast<double>(tb.denom);
-}();
-static uint64_t g_time_start = mach_absolute_time();
-
-void gpr_time_init(void) { gpr_precise_clock_init(); }
-
-static gpr_timespec now_impl(gpr_clock_type clock) {
-  gpr_timespec now;
-  struct timeval now_tv;
-  double now_dbl;
-
-  now.clock_type = clock;
-  switch (clock) {
-    case GPR_CLOCK_REALTIME:
-      // gettimeofday(...) function may return with a value whose tv_usec is
-      // greater than 1e6 on iOS The case is resolved with the guard at end of
-      // this function.
-      gettimeofday(&now_tv, nullptr);
-      now.tv_sec = now_tv.tv_sec;
-      now.tv_nsec = now_tv.tv_usec * 1000;
-      break;
-    case GPR_CLOCK_MONOTONIC:
-      // Add 5 seconds arbitrarily: avoids weird conditions in gprpp/time.cc
-      // when there's a small number of seconds returned.
-      now_dbl = 5.0e9 +
-                ((double)(mach_absolute_time() - g_time_start)) * g_time_scale;
-      now.tv_sec = (int64_t)(now_dbl * 1e-9);
-      now.tv_nsec = (int32_t)(now_dbl - ((double)now.tv_sec) * 1e9);
-      break;
-    case GPR_CLOCK_PRECISE:
-      gpr_precise_clock_now(&now);
-      break;
-    case GPR_TIMESPAN:
-      abort();
-  }
-
-  // Guard the tv_nsec field in valid range for all clock types
-  while (GPR_UNLIKELY(now.tv_nsec >= 1e9)) {
-    now.tv_sec++;
-    now.tv_nsec -= 1e9;
-  }
-  while (GPR_UNLIKELY(now.tv_nsec < 0)) {
-    now.tv_sec--;
-    now.tv_nsec += 1e9;
-  }
-
-  return now;
-}
-#endif
 
 gpr_timespec (*gpr_now_impl)(gpr_clock_type clock_type) = now_impl;