[breaking] remove alias exports `DefaultContext` and `EyreContext`

The following aliasing exports are unused and can be removed:

```rust
#[cfg(feature = "anyhow")]
#[doc(hidden)]
pub use DefaultHandler as DefaultContext;
#[cfg(feature = "anyhow")]
#[doc(hidden)]
pub use EyreHandler as EyreContext;
```

The aliases might be imported downstream, making this a breaking change.
