Remove the old NTPSymmetric model

It is no longer in use by any modules. It has been superseded by
NTPHeader.

lib/rex/proto/ntp/modes.rb

@@ -98,25 +98,6 @@ def records
     end
   end
 
-  class NTPSymmetric < BinData::Record
-    alias size num_bytes
-    endian :big
-    bit2   :li
-    bit3   :version, initial_value: 3
-    bit3   :mode
-    uint8  :stratum
-    uint8  :poll
-    uint8  :precision
-    uint32 :root_delay
-    uint32 :root_dispersion
-    uint32 :reference_id
-    uint64 :reference_timestamp
-    uint64 :origin_timestamp
-    uint64 :receive_timestamp
-    uint64 :transmit_timestamp
-    rest   :payload
-  end
-
   def ntp_control(version, operation, payload = nil)
     n = NTPControl.new
     n.version = version