checker: more default field fixes

vlib/net/html/data_structures.v

@@ -5,7 +5,7 @@ struct Stack {
 	null_element int = C.INT_MIN
 mut:
 	elements     []int
-	size         int = 0
+	size         int
 }
 
 fn (stack Stack) is_null(data int) bool {
@@ -44,7 +44,7 @@ fn (mut stack Stack) push(item int) {
 struct BTree {
 mut:
 	all_tags     []Tag
-	node_pointer int = 0
+	node_pointer int
 	childrens    [][]int
 	parents      []int
 }

vlib/net/html/parser.v

@@ -7,11 +7,11 @@ mut:
 	current_tag      &Tag
 	open_tag         bool = false
 	open_code        bool = false
-	open_string      int = 0
+	open_string      int
 	open_comment     bool = false
 	is_attribute     bool = false
-	opened_code_type string = ''
-	line_count       int = 0
+	opened_code_type string
+	line_count       int
 	lexeme_builder   string
 	code_tags        map[string]bool = {
 		'script': true

vlib/net/html/tag.v

@@ -15,7 +15,7 @@ mut:
 	attributes         map[string]string // attributes will be like map[name]value
 	last_attribute     string
 	parent             &Tag = C.NULL
-	position_in_parent int = 0
+	position_in_parent int
 	closed             bool = false
 	close_type         CloseTagType = .in_name
 }