Fix doc(hidden) errors.

[nnethercote]

Compiling with cargo 1.63.0-nightly (3f052d8ee 2022-05-12) I get lots
of errors like this:

error: `#[doc(hidden)]` is ignored on trait impl items
    --> src/header/map.rs:3296:9
     |
3296 |         #[doc(hidden)]
     |         ^^^^^^^^^^^^^^ help: remove this attribute
     |
note: the lint level is defined here
    --> src/lib.rs:161:9
     |
161  | #![deny(warnings, missing_docs, missing_debug_implementations)]
     |         ^^^^^^^^
     = note: `#[deny(unused_attributes)]` implied by `#[deny(warnings)]`
     = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
     = note: whether the impl item is `doc(hidden)` or not entirely depends on the corresponding trait item

This commit removes all these unnecessary attributes so that http compiles
again with Nightly.