Remove bogus assertion

Remove a bogus assertion in FamInst.newFamInst
(There is a comment to explain.)

This fixes Trac #16112.

compiler/typecheck/FamInst.hs

@@ -158,8 +158,10 @@ newFamInst :: FamFlavor -> CoAxiom Unbranched -> TcM FamInst
 -- Freshen the type variables of the FamInst branches
 newFamInst flavor axiom@(CoAxiom { co_ax_tc = fam_tc })
   = ASSERT2( tyCoVarsOfTypes lhs `subVarSet` tcv_set, text "lhs" <+> pp_ax )
-    ASSERT2( tyCoVarsOfType  rhs `subVarSet` tcv_set, text "rhs" <+> pp_ax )
     ASSERT2( lhs_kind `eqType` rhs_kind, text "kind" <+> pp_ax $$ ppr lhs_kind $$ ppr rhs_kind )
+    -- We used to have an assertion that the tyvars of the RHS were bound
+    -- by tcv_set, but in error situations like  F Int = a that isn't
+    -- true; a later check in checkValidFamInst rejects it
     do { (subst, tvs') <- freshenTyVarBndrs tvs
        ; (subst, cvs') <- freshenCoVarBndrsX subst cvs
        ; dflags <- getDynFlags