See elastic/apm#55

This also allows us to get rid of a hack we used to pass the API method
and an unserialized version of the body to the instrumentation. Neither
is used anymore, so we can skip that part of the instrumentation.

This does also mean that we stop capturing the body for some methods,
specifically `update` and `delete_by_query`.

closes elastic#419

using params=None never made sense here