html/template: remove unused mode field on Tree struct

While I was reviewing the relevant code for parsing of the HTML syntax
that golang uses. I found that this field wasn't being used anywhere in
the code. So it's a bit useless to have this field it here. It seemed
like that this field was replaced by the non-private field `Mode` but
the old one was never removed.

src/text/template/parse/parse.go

@@ -32,7 +32,6 @@ type Tree struct {
 	treeSet    map[string]*Tree
 	actionLine int // line of left delim starting action
 	rangeDepth int
-	mode       Mode
 }
 
 // A mode value is a set of flags (or 0). Modes control parser behavior.