Omit unused object_boxed from vtable in old no-std rustc

[dtolnay]

In this configuration, StdError refers to a separate trait that is private to anyhow, so the From impls are statically unusable downstream.

anyhow/src/lib.rs

Lines 281 to 282 in 213a9c2

	#[cfg(all(not(feature = "std"), anyhow_no_core_error))]
	trait StdError: Debug + Display {