fix: too many constructor params for HsExpr.MG

Building against ghc-7.8.2 on OSX, I got a compile error:

    [ 2 of 11] Compiling Info             ( src/Info.hs, dist/build/hdevtools/hdevtools-tmp/Info.o )

    src/Info.hs:134:59:
        Constructor ‘GHC.MG’ should have 3 arguments, but has been given 4
        In the pattern: GHC.MG _ _ typ _
        In the pattern: GHC.FunBind {GHC.fun_matches = GHC.MG _ _ typ _}
        In the pattern:
          GHC.L spn (GHC.FunBind {GHC.fun_matches = GHC.MG _ _ typ _})

According to [the documentation for `HsExpr.MatchGroup` in ghc-7.8.2][1] `MG` only takes 3
parameters:

```haskell
data MatchGroup id body
  = MG { mg_alts    :: [LMatch id body]  -- The alternatives
       , mg_arg_tys :: [PostTcType]      -- Types of the arguments, t1..tn
       , mg_res_ty  :: PostTcType  }     -- Type of the result, tr
     -- The type is the type of the entire group
     --      t1 -> ... -> tn -> tr
     -- where there are n patterns
  deriving (Data, Typeable)
```

So I fixed the bug by removing the extra ignored parameter, and now it
compiles fine.

  [1]: https://www.haskell.org/ghc/docs/7.8.2/html/libraries/ghc-7.8.2/HsExpr.html