Remove redundant Backtrace provide_ref from ContextError

[dtolnay]

Back when this line was added in 46d3d2c, it was necessary in order for the following program to successfully obtain the requested backtrace:

#![feature(error_generic_member_access)]

use anyhow::anyhow;
use std::backtrace::Backtrace;

fn main() {
    let error = anyhow!("...").context("...");
    (&*error).request_ref::<Backtrace>().unwrap();
}

However #268 made it redundant, as now self.error.provide(demand) has become a call to:

anyhow/src/error.rs

Lines 534 to 536 in cccc785

	fn provide<'a>(&'a self, demand: &mut Demand<'a>) {
	unsafe { ErrorImpl::provide(self.inner.by_ref(), demand) }
	}

which calls:

anyhow/src/error.rs

Lines 911 to 916 in cccc785

	unsafe fn provide<'a>(this: Ref<'a, Self>, demand: &mut Demand<'a>) {
	if let Some(backtrace) = &this.deref().backtrace {
	demand.provide_ref(backtrace);
	}
	Self::error(this).provide(demand);
	}