removed obsolete code

lib/single-file/doc-helper.js

@@ -29,9 +29,6 @@ this.docHelper = this.docHelper || (() => {
 	const INPUT_VALUE_ATTRIBUTE_NAME = "data-single-file-value";
 	const SHEET_ATTRIBUTE_NAME = "data-single-file-sheet";
 	const IGNORED_REMOVED_TAG_NAMES = ["NOSCRIPT", "DISABLED-NOSCRIPT", "META", "LINK", "STYLE", "TITLE", "TEMPLATE", "SOURCE", "OBJECT"];
-	const MASK_TAGNAME = "singlefile-mask";
-	const BACKDROP_THRESHOLD_SIZE = .95;
-	const BACKDROP_THRESHOLD_ZINDEX = 999;
 	const REGEXP_SIMPLE_QUOTES_STRING = /^'(.*?)'$/;
 	const REGEXP_DOUBLE_QUOTES_STRING = /^"(.*?)"$/;
 	const FONT_WEIGHTS = {