Remove Ruby <2.5 optimized Database#synchronize method

Sequel supports old Ruby versions in order to make it
easier to transition applications from old Ruby versions
to modern Ruby versions, but there is no reason to support
optimized versions of methods for old Ruby versions.

lib/sequel/database/connecting.rb

@@ -270,28 +270,20 @@ def single_threaded?
       @single_threaded
     end
 
-    if RUBY_ENGINE == 'ruby' && RUBY_VERSION < '2.5'
-    # :nocov:
-      def synchronize(server=nil)
-        @pool.hold(server || :default){|conn| yield conn}
-      end
-    # :nocov:
-    else
-      # Acquires a database connection, yielding it to the passed block. This is
-      # useful if you want to make sure the same connection is used for all
-      # database queries in the block.  It is also useful if you want to gain
-      # direct access to the underlying connection object if you need to do
-      # something Sequel does not natively support.
-      #
-      # If a server option is given, acquires a connection for that specific
-      # server, instead of the :default server.
-      #
-      #   DB.synchronize do |conn|
-      #     # ...
-      #   end
-      def synchronize(server=nil, &block)
-        @pool.hold(server || :default, &block)
-      end
+    # Acquires a database connection, yielding it to the passed block. This is
+    # useful if you want to make sure the same connection is used for all
+    # database queries in the block.  It is also useful if you want to gain
+    # direct access to the underlying connection object if you need to do
+    # something Sequel does not natively support.
+    #
+    # If a server option is given, acquires a connection for that specific
+    # server, instead of the :default server.
+    #
+    #   DB.synchronize do |conn|
+    #     # ...
+    #   end
+    def synchronize(server=nil, &block)
+      @pool.hold(server || :default, &block)
     end
 
     # Attempts to acquire a database connection.  Returns true if successful.