[ot-layout] Don't remove_output() before reverse substitution

No need anymore, because of new swap_buffers() semantics.

Just assert instead.

src/hb-buffer.cc

@@ -281,16 +281,6 @@ hb_buffer_t::add_info (const hb_glyph_info_t &glyph_info)
 }
 
 
-void
-hb_buffer_t::remove_output ()
-{
-  have_output = false;
-  have_positions = false;
-
-  out_len = 0;
-  out_info = info;
-}
-
 void
 hb_buffer_t::clear_output ()
 {

src/hb-buffer.hh

@@ -209,7 +209,6 @@ struct hb_buffer_t
   HB_INTERNAL void guess_segment_properties ();
 
   HB_INTERNAL void swap_buffers ();
-  HB_INTERNAL void remove_output ();
   HB_INTERNAL void clear_output ();
   HB_INTERNAL void clear_positions ();
 

src/hb-ot-layout.cc

@@ -1900,10 +1900,8 @@ apply_string (OT::hb_ot_apply_context_t *c,
   else
   {
     /* in-place backward substitution/positioning */
-    if (Proxy::table_index == 0u)
-      buffer->remove_output ();
+    assert (!buffer->have_output);
     buffer->idx = buffer->len - 1;
-
     apply_backward (c, accel);
   }
 }