Bug 1700801 - Remove stale test annotation of test_bug756984 which passes with fission-xorigin. r=layout-reviewers,TYLin

Differential Revision: https://phabricator.services.mozilla.com/D110603

layout/generic/test/mochitest.ini

@@ -80,7 +80,6 @@ support-files = bug633762_iframe.html
 [test_bug756984.html]
 fail-if =
     os == 'linux' && os_version == '18.04'  # Bug 1600208 permafail on ubuntu1804
-    xorigin
 [test_bug784410.html]
 [test_bug785324.html]
 [test_bug791616.html]