Remove HsEqTy and XEqTy

After commit d650729f, the
`HsEqTy` constructor of `HsType` is essentially dead code. Given that
we want to remove `HsEqTy` anyway as a part of #10056 (comment:27),
let's just rip it out.

Bumps the haddock submodule.

Test Plan: ./validate

Reviewers: goldfire, bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #10056

Differential Revision: https://phabricator.haskell.org/D4876