older clients require 'separate' on filtered decks

the deckconf['new']['separate' property is unused, but older code
assumes filtered['separate'] exists, even though it doesn't do anything
with it

https://anki.tenderapp.com/discussions/beta-testing/1868-anki-2124-beta#comment_48240796

rslib/src/deckconf.rs

@@ -55,10 +55,6 @@ pub struct NewConf {
     #[serde(deserialize_with = "default_on_invalid")]
     pub(crate) per_day: u32,
 
-    // unused, can remove in the future
-    #[serde(default)]
-    separate: bool,
-
     #[serde(flatten)]
     other: HashMap<String, Value>,
 }
@@ -165,7 +161,6 @@ impl Default for NewConf {
             ints: NewCardIntervals::default(),
             order: NewCardOrder::default(),
             per_day: 20,
-            separate: true,
             other: Default::default(),
         }
     }

rslib/src/decks.rs

@@ -55,8 +55,7 @@ mod dynfix {
                 );
             }
 
-            // remove some obsolete keys
+            // remove an obsolete key
-            map.remove("separate");
             map.remove("return");
 
             let rest = Value::Object(map);
@@ -116,6 +115,10 @@ pub struct FilteredDeck {
     resched: bool,
     terms: Vec<FilteredSearch>,
 
+    // unused, but older clients require its existence
+    #[serde(default)]
+    separate: bool,
+
     // old scheduler
     #[serde(default, deserialize_with = "default_on_invalid")]
     delays: Option<Vec<f32>>,