[ruby/json] Drop compatibility for missing Time#tv_nsec (Ruby 1.8)

https://github.com/ruby/json/commit/b240bde402

Co-authored-by: Jean Boussier <jean.boussier@gmail.com>

ext/json/lib/json/add/time.rb

@@ -10,11 +10,7 @@ class Time
     if usec = object.delete('u') # used to be tv_usec -> tv_nsec
       object['n'] = usec * 1000
     end
-    if method_defined?(:tv_nsec)
-      at(object['s'], Rational(object['n'], 1000))
-    else
-      at(object['s'], object['n'] / 1000)
-    end
+    at(object['s'], Rational(object['n'], 1000))
   end
 
   # Methods <tt>Time#as_json</tt> and +Time.json_create+ may be used
@@ -34,13 +30,10 @@ class Time
   #    # => 2023-11-25 11:00:56.472846644 -0600
   #
   def as_json(*)
-    nanoseconds = [ tv_usec * 1000 ]
-    respond_to?(:tv_nsec) and nanoseconds << tv_nsec
-    nanoseconds = nanoseconds.max
     {
       JSON.create_id => self.class.name,
       's'            => tv_sec,
-      'n'            => nanoseconds,
+      'n'            => tv_nsec,
     }
   end