client.py: remove the old "server.privkey" fallback

client.py: remove the old "server.privkey" fallback

For a brief while (in between releases 1.9 and 1.10, specifically from
revision bc21726 on 12-Mar-2012, until bf416af on 10-Jun-2012), the new
introducer code stored its node key in NODEDIR/private/server.privkey .
After that point, it was updated to store this key in
NODEDIR/private/node.privkey instead. Fallback code was added to read
from the old location if present (so that folks using development
versions could keep their node keys after the bf416af change).

This patch removes the fallback code. If you have a node which was run
under a version of Tahoe within this range, you need to manually update
your node by running:

 mv NODEDIR/private/server.privkey NODEDIR/private/node.privkey

and then restart the node. If you accidentally start an older node with
code after this patch, it will create a new key (and other peers will
think a new server has appeared). You can either stick with the new key,
or use the command above to switch back to the old key.

See docs/nodekeys.rst (not yet written) for details about the node key
and how it is used.