Remove ignored inline attributes from function prototypes

[oherrala]

Attributes that are in function prototypes are ignored and emit a compiler warning. This pull request should not have any effect except removing the compiler warnings:

warning: `#[inline]` is ignored on function prototypes
   --> /Users/oherrala/.cargo/registry/src/index.crates.io-6f17d22bba15001f/subtle-2.5.0/src/lib.rs:282:5
    |
282 |     #[inline]
    |     ^^^^^^^^^
    |
    = note: `#[warn(unused_attributes)]` on by default

warning: `#[inline]` is ignored on function prototypes
   --> /Users/oherrala/.cargo/registry/src/index.crates.io-6f17d22bba15001f/subtle-2.5.0/src/lib.rs:425:5
    |
425 |     #[inline]
    |     ^^^^^^^^^

warning: `#[inline]` is ignored on function prototypes
   --> /Users/oherrala/.cargo/registry/src/index.crates.io-6f17d22bba15001f/subtle-2.5.0/src/lib.rs:612:5
    |
612 |     #[inline]
    |     ^^^^^^^^^

warning: `subtle` (lib) generated 3 warnings

[AaronFeickert]

Might it be useful to keep the attributes as a helpful indication for implementers and silence the warnings, as in #121?