reader rls: remove redundant initial value

This initial value is weird and None works the same way so use that.

src/reader/reader.rs

@@ -590,7 +590,7 @@ fn new() -> Self {
         Self {
             last_cmd: None,
             yank_len: 0,
-            completion_action: Some(CompletionAction::InsertedUnique),
+            completion_action: None,
             finished: false,
             nchars: None,
         }