subtype: Try removing ill-motivated code-path (#61942)

The code-path I'm removing in this PR doesn't make much sense to me -
the comment that goes before it makes a true statement, but this is not
the place that should be detecting this. I suspect that this code path
was necessary earlier in the life of the type system but was working
around a bug that has since been fixed. If there are remaining cases
that trigger this, it would be good to find them, since test/subtype.jl
doesn't have any. It would be nice to remove it because it's
structurally a bit odd - it compares the `ub` and the `lb` from a
non-existential typevar, but subtyping doesn't adjust these, so if they
are in fact equal, then probably unionall introduction should have
figured that out upon introduction, rather than recursing down this path
deep in the leaves. Additionally, one of the code paths (but not the
other) only triggers on PARAM_INVARIANT, which is not supposed to have a
semantic affect outside of the diagonal rule.

So let's find out if there is any code that depends on this. If so, we
can see if there's a structurally better way to handle this case, if
not, we can just merge this.

While we're at it, also pull up the lookup into the caller so we don't
have to duplicate that work.