oasis.eclass: mark as @DEAD (last-rite eclass)

Removal on 2021-04-05 when the rest of the Oasis packages, including
Oasis (dev-ml/oasis) is	removed.

There's no point in keeping the eclass when Oasis itself is gone and nothing
should be able to use this right now anyway due to the large number of packages
which have been removed and conflicts with e.g. jbuilder and Dune.

(Even if something IS using this, they *really* shouldn't be by now, and the vast
majority - if not all - of their packages would have been from outside ::gentoo,
as Oasis and friends have not been usable in the tree for at least a year.)

Please port to newer opam or dune.

Bug: https://bugs.gentoo.org/775785
Bug: https://bugs.gentoo.org/497044
Bug: https://bugs.gentoo.org/637826