Remove wrong ASSERT.

In the following example, there is no extra-constraints wildcard, so just
return the annotated constraints. At the same time, the inferred constraints
will also include (Eq a), so the ASSERT was incorrect.

f :: Eq a => _ -> a -> Int
f True x = if x == x then 1 else 2
Fix: constraints were forgotten when there was no extra-constraints wildcard

The `Eq a` constraint in the example below was forgotten:

    f :: Eq a => _ -> a -> Int
    f True x = if x == x then 1 else 2

Remove obsolete code and minimise the diff at the same time. Also update
changed error messages.