Remove rel, target from attributes, which don’t work

They are currently not allowed by GH.

Author: wooorm

lib/schema.js

@@ -113,7 +113,6 @@ export const defaultSchema = {
       'open',
       'prompt',
       'readOnly',
-      'rel',
       'rev',
       'rowSpan',
       'rows',
@@ -126,7 +125,6 @@ export const defaultSchema = {
       'start',
       'summary',
       'tabIndex',
-      'target',
       'title',
       'useMap',
       'vAlign',