chore!: remove alias exports DefaultContext and EyreContext

`DefaultContext` and `EyreContext` exist as
alias exports enabled by the `anyhow` feature.

However, they are unused within `eyre`; neither
are handlers of such names exposed by `anyhow`.

This changeset removes both handler aliases.

Closes [eyre-rs#135][135].

[135]: eyre-rs#135

BREAKING CHANGE: The alias exports are undocumented
                 but nevertheless part of the
                 crate's public API.

eyre/src/lib.rs

@@ -389,12 +389,6 @@ pub use eyre as format_err;
 pub use eyre as anyhow;
 use once_cell::sync::OnceCell;
 use ptr::OwnedPtr;
-#[cfg(feature = "anyhow")]
-#[doc(hidden)]
-pub use DefaultHandler as DefaultContext;
-#[cfg(feature = "anyhow")]
-#[doc(hidden)]
-pub use EyreHandler as EyreContext;
 #[doc(hidden)]
 pub use Report as ErrReport;
 /// Compatibility re-export of `Report` for interop with `anyhow`