Remove GPX output format

[TheMarex]

Responses by the server should handled in JSON only.

• The response of most plugins has no meaningful mapping to GPX
• The current implementation of GPX is a hack that involves rendering JSON first and then transforming that into XML
• It creates an unneeded complexity in the response rendering code
• It is off little use in the current form GPX File Export #1180

Since this breaks the API we should aim for 5.0

[stevage]

What does this mean for the current way GPX's are generated in OSRM-web? Is the "hack" that you refer to done in the client already, and hence this isn't needed?

[TheMarex]

@stevage no gpx is currently generated at the server. But it is pretty trivial for the client to transform JSON to XML if needed.

[TheMarex]

This was implemented as part of #1851