Remove dead code in extra_field/generic.rb (==).

From what I can tell this was erroneously copied out of extra_field.rb
during a refactor. It attempts to compare a non-existent Hash that used
to be inherited before the refactor. If this code had been left within
ExtraField it would make more sense, but as it's not needed there either
let's just remove it.

See 20d79fe for the refactor.

lib/zip/extra_field/generic.rb

@@ -22,15 +22,6 @@ def initial_parse(binstr)
       [binstr[2, 2].unpack1('v'), binstr[4..-1]]
     end
 
-    def ==(other)
-      return false if self.class != other.class
-
-      each do |k, v|
-        return false if v != other[k]
-      end
-      true
-    end
-
     def to_local_bin
       s = pack_for_local
       self.class.const_get(:HEADER_ID) + [s.bytesize].pack('v') << s