This fixes validate.

37445780 (D202) mentions: "This patch
also wires-in Maybe data type".

A conflicting definition of a wired-in type in a .hsig file doesn't seem
to cause compilation to fail. This is probably a bug, but a small one.
Since SPJ in ffc21506 swept this under
the rug, by removing `data Bool a b c d = False` from tcfail220.hsig,
I'm going to do the same here.

D1098 touches these files, so ezyang can decide whether this problem
warrants fixing when doing a rebase.

Differential Revision: https://phabricator.haskell.org/D1208