Let save_self_keypair() also remove a previously used key

The imex code was deleting the key if it was already used.  Turns out
none of the callers would not want this behaviour, so let's just
handle this case generally.

src/imex.rs

@@ -315,13 +315,6 @@ fn set_self_key(
     ensure!(self_addr.is_some(), "Missing self addr");
     let addr = EmailAddress::new(&self_addr.unwrap_or_default())?;
 
-    // XXX maybe better make dc_key_save_self_keypair delete things
-    sql::execute(
-        context,
-        &context.sql,
-        "DELETE FROM keypairs WHERE public_key=? OR private_key=?;",
-        params![public_key.to_bytes(), private_key.to_bytes()],
-    )?;
     let (public, secret) = match (public_key, private_key) {
         (Key::Public(p), Key::Secret(s)) => (p, s),
         _ => bail!("wrong keys unpacked"),