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).