google-api-go-client/googleapi: remove unused interface.
google-api-go-client/googleapi: remove unused interface.

Lengther was used by code that has now been deleted.  Remove it.

Change-Id: If1f3fab50ede1bd1b4190552a1160c8f5e772c9c
Reviewed-on: https://code-review.googlesource.com/3970
Reviewed-by: Dave Day <4cba7a21e1f1fc896442779dcb55aeda6df97dfe@golang.org>

google/google-api-go-client,google/google-api-go-client,googleapis/google-api-go-client,googleapis/google-api-go-client