server/keymgmt.c: Remove redundant error reports.

Any I/O errors encountered while reading the key file will have been
reported already.  In the same terms.

Previously, the final `die' message wasn't actually redundant, because
the earlier warnings got swallowed if we were going to run as a daemon.
It's obviously wrong that this happened, and the underlying cause has
been fixed properly now, so this isn't a concern any more.