Fix another egregious regression from 8a033b9f3 Add undo

The first character in the commandline could not be deleted with ^D.

src/reader.cpp

@@ -1210,8 +1210,6 @@ void reader_data_t::delete_char(bool backward) {
     }
     size_t pos_end = pos;
 
-    if (el->position() == 0) return;
-
     // Fake composed character sequences by continuing to delete until we delete a character of
     // width at least 1.
     int width;