Remove unneeded feature declarations

[joshuawarner32]

This allows this crate to compile on stable rust, 1.48. Previously, I was seeing these errors:

error[E0554]: `#![feature]` may not be used on the stable release channel
  --> /home/joshw/.cargo/registry/src/github.com-1ecc6299db9ec823/doh-dns-0.2.0/src/lib.rs:59:1
   |
59 | #![feature(proc_macro_hygiene)]
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error[E0554]: `#![feature]` may not be used on the stable release channel
  --> /home/joshw/.cargo/registry/src/github.com-1ecc6299db9ec823/doh-dns-0.2.0/src/lib.rs:60:1
   |
60 | #![feature(stmt_expr_attributes)]
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

It's unclear to me why the code originally required these features? If they were stabilized I would expect a different error, about the feature decl not beeing needed.

[joshuawarner32]

For reference, these are the tracking issues for the two features:

• Tracking issue for procedural macros and "hygiene 2.0" rust-lang/rust#54727
• Tracking issue for stmt_expr_attributes: Add attributes to expressions, etc. rust-lang/rust#15701

As of writing, they're both open, which makes me suspect either this library no longer requires these features, or it never did (I haven't investigated to understand which it is / why)