Revert "Merge pull request #25880 from ryandv/fix_performance_regression_in_timewithzone_to_time"

Turns out trying to cache on localtime with arguments is too hard
so we'll do it on DateAndTime::Compatibility#to_time instead.

This reverts commit 3132fa6b, reversing
changes made to 6949f8e5.

activesupport/lib/active_support/time_with_zone.rb

@@ -80,7 +80,7 @@ def in_time_zone(new_zone = ::Time.zone)
 
     # Returns a <tt>Time</tt> instance of the simultaneous time in the system timezone.
     def localtime(utc_offset = nil)
-      @localtime ||= utc.getlocal(utc_offset)
+      utc.getlocal(utc_offset)
     end
     alias_method :getlocal, :localtime