This patch fixes Trac #15778 by removing a zonkTcTyVarToTyVar
from tcHsSigType.

Nww that a pattern-bound type variable can refer to a type, it's
obvoiusly wrong to expect it to be a TyVar!  Moreover, that zonk
is entirely unnecessary.

I added a new Note [Type variables in the type environment]
in TcRnTypes