Remove the support for the legacy config format

[szabgab]

Problem

mdbook still supports the legacy book.toml format that, if I am not mistaken, was deprecated in 2017.
(git blame src/config.rs puts the warning message to a commit on 2017-11-12)

Proposed Solution

Remove that support an create a new release as mdbook version 0.5 to indicate the breaking change.

Notes

No response

[ehuss]

Would be happy to get a PR for this.