Remove support for stub-* files in the manifest

[foolip]

The manifest support was added in commit 2650e63,
ensuring they aren't treated as tests.

Part 2 of web-platform-tests/rfcs#27.

[foolip]

Thanks for the review, @jgraham. When verifying that an incremental update would work I found that it didn't and pushed some changes to deal with that.

Is that fine, or should we bump the manifest version?

[jgraham]

Oh we don't handle a whole test type being dropped. Hmm, seems a shame to leave that code in until we next bump the manifest version, but maybe not too bad. Add a comment indicating it should be removed when the manifest version is higher than whatever the current version is.

@gsnedders may disagree of course.

[gsnedders]

It seems deliberate that we don't handle test types being removed, though we are pretty inconsistent with validating the JSON conforms to the expected schema.

Can you add a TODO(MANIFESTv7) to the comment so we remember to drop it when we bump the version?

[foolip]

@gsnedders do you want to review again?

[foolip]

@gsnedders merging now, please comment after the fact if you don't like what I did.