remove /etc/bind.keys

the built-in trust anchors in named and delv are sufficent for validation. named still needs to be able to load trust anchors from a bind.keys file for testing purposes, but it doesn't need to be the default behavior.

we now only load trust anchors from a file if explicitly specified via the "bindkeys-file" option in named or the "-a" command line argument to delv. documentation has been cleaned up to remove references to /etc/bind.keys.

Closes #3850 (closed)