remove legacy code paths from isElementInstance()

- nsIDOMElement was removed in mozilla61 in Bug 1455674
- call to importGlobalProperties was made unnecessary in mozilla65 in
  Bug 1489301 part 4 (see also: Bug 1501127)

extension/lib/markable-elements.coffee

@@ -3,12 +3,7 @@
 utils = require('./utils')
 viewportUtils = require('./viewport')
 
-_Element = Ci.nsIDOMElement
-if _Element
-  isElementInstance = (el) -> el instanceof _Element
-else
-  Cu.importGlobalProperties(['Element'])
-  isElementInstance = (el) -> Element.isInstance(el)
+isElementInstance = (el) -> Element.isInstance(el)
 
 MIN_TEXTNODE_SIZE = 4