Remove useless `Option<Ident>` arg.

`FmtVisitor::visit_mac` has an `Option<Ident>` arg which is always
either `None` or `Some(kw::Empty)`, because `ItemKind::MacCall` always
has an empty ident. This value is passed through various functions until
it reaches `rewrite_macro_name`, which treats `None` and
`Some(kw::Empty)` the same.

In other words, the argument is useless. This commit removes it. There
is no change in behaviour. The commit also changes a few `symbol::Ident`
occurrences to `Ident` in `macros.rs`; `Symbol` is imported in that file
so `Ident` might as well be, too.

(This is a good example of why it's a bad idea for `Itemt` to have an
`ident` field when various item kinds don't have an identifier. It's
easy to get confused when "empty identifier" is used to mean "no
identifier". This will be fixed in a subsequent commit.)

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