In TcMType.writeMetaTyVarRef we have an assertion
check, level_check_ok, that the type being written
to a unification variable is not deeper than the level
of the unification varaible itself.

This check used to have a special case for fmv/fsk
flatten vars, but this commit changed fmv/fsks to have
an ordinary level number:

    commit 2bbdd00c
    Author: Simon Peyton Jones <simonpj@microsoft.com>
    Date:   Fri May 18 08:43:11 2018 +0100

    Orient TyVar/TyVar equalities with deepest on the left

So we can delete the isFlattenTyVar special case from
the level_check_ok assertion.  Simpler, less ad hoc.