Remove two problematic XML linter unit tests

Summary:
Fixes T7215. See D13991. These test cases have been failing intermittently for a while.

I think the XML stuff (which we don't control) changed where it raises these warnings: an old version raised them at the end of the attribute, while the new version raises them at the beginning of the attribute. Not totally 100% sure about this since installing multiple versions is fairly inconvenient, but as far as I know both versions raise the warnings, just at different character offsets.

We could do various things to fix these tests (allow the warning to raise at any character, skip the tests based on versions, etc) but I think it's easier to just remove the tests. They don't seem valuable.

Test Plan: Tests failed prior to change; now pass.

Reviewers: chad, joshuaspence

Reviewed By: joshuaspence

Maniphest Tasks: T7215

Differential Revision: https://secure.phabricator.com/D13992