Remove Clock::NtpToMs.

This helper method does not belong to the Clock class. Also, it's simple enough that it's not needed.

Bug: webrtc:11327
Change-Id: I95a33f08fd568b293b591171ecaf5e7aef8d413c
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/214123
Reviewed-by: Henrik Andreassson <henrika@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Reviewed-by: Minyue Li <minyue@webrtc.org>
Commit-Queue: Paul Hallak <phallak@google.com>
Cr-Commit-Position: refs/heads/master@{#33652}

system_wrappers/include/clock.h

@@ -45,11 +45,6 @@ class RTC_EXPORT Clock {
   // Retrieve an NTP absolute timestamp in milliseconds.
   virtual int64_t CurrentNtpInMilliseconds() = 0;
 
-  // Converts an NTP timestamp to a millisecond timestamp.
-  static int64_t NtpToMs(uint32_t seconds, uint32_t fractions) {
-    return NtpTime(seconds, fractions).ToMs();
-  }
-
   // Returns an instance of the real-time system clock implementation.
   static Clock* GetRealTimeClock();
 };