Use PROCESS_CPUTIME_ID for cpu time metrics

To measure ruby cpu time, we should use PROCESS_CPUTIME_ID instead of
THREAD_CPUTIME_ID in our environments.

lib/gitlab/metrics/system.rb

@@ -57,18 +57,10 @@ module Gitlab
         end
       end
 
-      # THREAD_CPUTIME is not supported on OS X
-      if Process.const_defined?(:CLOCK_THREAD_CPUTIME_ID)
-        def self.cpu_time
-          Process
-            .clock_gettime(Process::CLOCK_THREAD_CPUTIME_ID, :float_second)
-        end
-      else
       def self.cpu_time
         Process
           .clock_gettime(Process::CLOCK_PROCESS_CPUTIME_ID, :float_second)
       end
-      end
 
       # Returns the current real time in a given precision.
       #