accounts/abi/bind/backends: remove nil and remote backends

The remote backend is superseded by ethclient.

The nil backend's stated purpose was to enable testing of
accounts/abi/bind. None of its methods actually worked. A much simpler
way to get a crashing backend is to simply pass nil as the backend. With
a one-line change to the generator (removing two explicit interface
assertions), passing nil actually works.

Removing these backends means that less changes are required later.