extension_methods directive is now obsolete.

extension_methods directive is now obsolete.
METHOD_OTHER type now cleanly handles unknown methods in squid.

Part 1:
  - removes the directive from squid default config.
  - removes the placeholder EXT methods enumerations
  - deprecates the code which previously handled config parsing.

part 2: (TODO)
 - needs to cleanup the remaining enumerated methods to reduce
   memory footprint