doc: remove references to old harmony flags

The following flags were removed from v8 and cannot be used anymore:
--harmony-scoping: https://codereview.chromium.org/1007783002
--harmony-strings: https://codereview.chromium.org/1024813002
--harmony-templates: https://codereview.chromium.org/1027593005

doc/iojs.1

@@ -101,16 +101,10 @@ If set to 1 then colors will not be used in the REPL.
         type: bool  default: false
   --harmony_numeric_literals (enable "harmony numeric literals")
         type: bool  default: true
-  --harmony_strings (enable "harmony string methods")
-        type: bool  default: true
-  --harmony_scoping (enable "harmony block scoping")
-        type: bool  default: true
   --harmony_classes (enable "harmony classes (implies block scoping & object literal extension)")
         type: bool  default: true
   --harmony_object_literals (enable "harmony object literal extensions")
         type: bool  default: true
-  --harmony_templates (enable "harmony template literals")
-        type: bool  default: true
   --compiled_keyed_generic_loads (use optimizing compiler to generate keyed generic load stubs)
         type: bool  default: false
   --pretenuring_call_new (pretenure call new)