Remove feature(use_extern_macros)

[dtolnay]

https://github.com/koute/stdweb/blob/3c8f62f06fc884778409ea79a9fb3f4f2a1c1972/src/lib.rs#L113-L116

This feature was stabilized as part of the macro-related stabilizations in Rust 1.30 so feature(use_extern_macros) is no longer needed.

Came up in https://github.com/bodil/typed-html/issues/6.

[koute]

Thanks for the reminder!

I was aware of this, however I did deliberately left it there for those still using old nightlies. I guess now would be a good enough time to get rid of it.

Fixed on master!