filters: Drop useless .open callbacks

filters: Drop useless .open callbacks

The cache filter .open has never done any useful work; since its
introduction in c10d126f, it appears to exist purely as copy-and-paste
from the cow filter (differing only in whether the readonly parameter
is massaged before passing on to next).

The error filter .open used to track a per-connection handle, but for
testing purposes, it was changed to use only global state in commit
b33ccbb8.

Drop these two .open callbacks in favor of using nbdkit's default
behavior, with no semantic change.  All other filters with an .open
callback either massage parameters passed to next, create a
per-connection handle, or both.

Signed-off-by: Eric Blake <eblake@redhat.com>