Remove overriding impls of deprecated `Error::description`.

[hkBst]

I propose to remove overriding impls of deprecated Error::description.

I naively proposed removing one such here as I was under the mistaken impression that it would delegate to Display::fmt, but it actually defaults to "description() is deprecated; use Display".

Deprecation happened 46 stable versions ago (in 1.42), or almost 6 years ago.

git grep 'fn description' | wc -l currently gives 72 results, which would need to be changed.

[cuviper]

Nominating for libs-api in particular, since this message is user-facing (if anyone is still using the deprecated method).

[programmerjake]

i didn't see any that looked like they were being used, but there is quite a lot of rust code that calls a description() method on github the first 2 pages have several uses of Error::description() afaict. at least two of them are published on crates.io.

that makes me think rust shouldn't make this change because it almost certainly breaks a lot of old code which we promised we'd keep working.