Remove spurious net6.0-windows7 folders

[idg10]

The Rx 6.0 preview.1 build has net6.0-windows7 folders underneath both build and buildTransitive that we suspect should not be present.

First, work out why they are there - it looks like the previous version had something equivalent. If that's true, work out what purpose they served. It's conceivable that they do actually need to be there. But if we don't need them, remove them.

[idg10]

Ah, turns out that these were introduced by #1442, but the reason Rx 5.0.0 doesn't have these folders is that #1442 was merged after the last release of Rx! (So although it was merged, it never shipped.)

As per the discussion in #1889 (comment) it's looking like we probably don't want to keep that change. Backing that out should have the side effect of removing these folders from the package.

[idg10]

Fixed in #1901