Remove fun_id, is_infix from FunBind, as they are now in Match

One of the changes D538 introduced is to add m_fun_id_infix to Match

data Match id body
  = Match {
        m_fun_id_infix :: (Maybe (Located id,Bool)),
          -- fun_id and fun_infix for functions with multiple equations
          -- only present for a RdrName. See note [fun_id in Match]
        m_pats :: [LPat id], -- The patterns
        m_type :: (Maybe (LHsType id)),
                                 -- A type signature for the result of the match
                                 -- Nothing after typechecking
        m_grhss :: (GRHSs id body)
  } deriving (Typeable)

This was done to track the individual locations and fixity of the fun_id for each of the defining equations for a function when there are more than one.

For example, the function (&&&) is defined with some prefix and some infix equations below.

(&&&  ) [] [] =  []
xs    &&&   [] =  xs
(  &&&  ) [] ys =  ys

This means that the fun_id / is_infix is now superfluous in the FunBind. This has not been removed as a potentially risky change just before 7.10 RC2, and so must be done after.

This ticket captures that task, which includes processing these fields through the renamer and beyond.