The Remove method for the keyring sliced the internal keys list
incorrectly when removing a key. This caused the wrong key to be removed
or sometimes multiple keys were removed. Additionally, if the key to be
removed was the last key, the method never returned.
Fixesgolang/go#13628
Change-Id: I0facbcb8f8b65709222067ce37ea26e3fb5ba8e8
Reviewed-on: https://go-review.googlesource.com/17870
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>