Remove check from Invitation:take()

I've checked the spots when `Invitation::new()` and `Invitation::take()`
are used and it seems like all spots are already correctly gated. So to
enable invitations via admin API even when invitations are otherwise
disabled, this check can be removed.

src/db/models/user.rs

@@ -319,8 +319,7 @@ impl Invitation {
     }
 
     pub fn take(mail: &str, conn: &DbConn) -> bool {
-        CONFIG.invitations_allowed()
+        match Self::find_by_mail(mail, &conn) {
-            && match Self::find_by_mail(mail, &conn) {
             Some(invitation) => invitation.delete(&conn).is_ok(),
             None => false,
         }