[morx] Remove mark_set from Insertion

text-rendering-tests test MORX-32 shows that for Insertion, an unset mark is treated
as mark set at 0.  This is unlike the Reordering lookup where un unset mark performs
nothing.

Fixes MORX-32.

src/hb-aat-layout-morx-table.hh

@@ -592,7 +592,6 @@ struct InsertionSubtable
 			     hb_aat_apply_context_t *c_) :
 	ret (false),
 	c (c_),
-	mark_set (false),
 	mark (0),
 	insertionAction (table+table->insertionAction) {}
 
@@ -608,7 +607,7 @@ struct InsertionSubtable
       hb_buffer_t *buffer = driver->buffer;
       unsigned int flags = entry->flags;
 
-      if (entry->data.markedInsertIndex != 0xFFFF && mark_set)
+      if (entry->data.markedInsertIndex != 0xFFFF)
       {
 	unsigned int count = (flags & MarkedInsertCount);
 	unsigned int start = entry->data.markedInsertIndex;
@@ -669,10 +668,7 @@ struct InsertionSubtable
       }
 
       if (flags & SetMark)
-      {
-	mark_set = true;
 	mark = buffer->out_len;
-      }
 
       return true;
     }
@@ -681,7 +677,6 @@ struct InsertionSubtable
     bool ret;
     private:
     hb_aat_apply_context_t *c;
-    bool mark_set;
     unsigned int mark;
     const UnsizedArrayOf<GlyphID> &insertionAction;
   };