keymgr: Remove unnecessary condition.
We don't need to special-case NotFound
errors because
ssh_key::PrivateKey::from_openssh
doesn't read from disk (so it
shouldn't be returning NotFound
errors in the first place).
Previously, this condition made sense because we were using
ssh_key::PrivateKey::read_openssh_file
instead of
ssh_key::PrivateKey::from_openssh
, which does read from disk. This
condition should have been removed in !1263 (merged) (the refactoring where we
switched to from_openssh
).
(Also, after #901 (closed) is implemented, we probably won't have an
Error::NotFound
error anymore).