[Mac] Should pass http/tests/images/png-partial-load-no-alpha.html
https://bugs.webkit.org/show_bug.cgi?id=97170

Unreviewed test expectations update.

* platform/mac/TestExpectations: Passing after r150340: remove the
Mac-specfic [ImageOnlyFailure] for this test.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@151010 268f45cc-cd09-0410-ab3c-d52691b4dbfc