remove unused constructors for error messages

[winitzki]

While working on https://github.com/dhall-lang/dhall-haskell/pull/2651 I noticed that several error message constructors were not used to create any error messages in the type-checker. Those constructors are:

• InvalidHandlerOutputType
• InvalidListElement
• MustUpdateARecord
• RecordTypeMismatch

In all those cases, the error description shows some example errors but those errors are reported via different error messages. There is no code anywhere in this repository that creates error values or raises exceptions using these constructors.

For example, it looks like InvalidListElement was intended to describe the type error in [1, True] : List Natural. However, Dhall reports the error "List elements should all have the same type" created via MismatchedListElements. The type error in [1, 2] : List Text is "Expression does not match type annotation".

I would like to remove all code around those unused error message constructors.