According to docs, local-repo is deprecated, so new-build shouldn't support it

[ezyang]

The docs say:

This can be used to access repositories on the local file system. However, the
layout of these local repositories is different from the layout of remote
repositories, and usage of these local repositories is deprecated.

But new-build supports it! So is it deprecated? If so let's remove it.

Feature was added in 0a2fb23

CC @edsko (who added the doc entry) @dcoutts

[dcoutts]

True, though we should also check that local repos do work ok in the new secure code path.

[phadej]

If nobody takes ownership (i.e. assigns to themselves, and promises to deliver this year) before Thursday 2019-12-05, I'll move this issue to 3.4.

[phadej]

Related #6359

[phadej]

remilestoning for 3.4