Remove hand-rolled format shorthand when we drop support for Rust < 1.58

[shepmaster]

write!(..., "{foo}") capturing foo stabilized in Rust 1.58. When we update MSRV past that, we can remove the code from the derive macro.

[shepmaster]

Haha, Rust is too clever:

note: to avoid ambiguity, format_args! cannot capture variables when the format string is expanded from a macro