This fixes scala#15692 and does not seem to break an existing compilation
tests. The problem with seen when it comes scala#15692 is that seen means
that type that has repeated types will get a lower size and this
incorrectly triggers the divergence check since some of the steps
involved less repeated types.

The seen logic was introduced in scala#6329 and the motivation was to deal
with F-bounds. Since not tests fail it not clear if this logic is still
needed to deal with F-bounds? If it is still needed we can add a test
and instead of removing the seen logic we can make it track only types
the appear as a bound and could cause infinite recursion instead of
tracking all.

[Cherry-picked c2ef180]

This got introduced, because we no longer check in Scala Next. I added an additional check in metals to make sure this doesn't happen anymore.