bind: use default rndc control channel

bind: use default rndc control channel

The present logic recreates what is already the default in bind [1], and writes
the rndc key twice to two different files. In addition, the rndc key is
regenerated every time bind is restarted. Simplify this by relying on the
default behaviour instead.

[1] https://bind9.readthedocs.io/en/latest/reference.html#controls-block-definition-and-usage

Signed-off-by: David Härdeman <[email protected]>