Remove seen from TypeSizeAccumulator

[eejbyfeldt]

This fixes #15692 and does not seem to break any existing compilation tests. The problem with seen logic is that give types with repeated types will get a lower size and this incorrectly triggers the divergence check since #15692 some of the steps involved less repeated types.

The seen logic was introduced in #6329 and the motivation was to deal with F-bounds. Since no 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 that fails 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.