Consider no longer implementing deprecated Error methods

[shepmaster]

In rust-lang/libs-team#615 (comment), the libs-team said:

Considering that [Error::]description has been deprecated for over 5 years, it's reasonable to assume it is no longer used much and it's therefore acceptable to [stop implementing Error::description and instead] return a lower-quality error message

If the standard library will start performing work to remove whatever existing implementations there are, perhaps we should likewise stop generating them. Might be a small compile time boost as well...