Remove Author.email field

For example "John Smith <john.smith@example.com>" and "John Smith <jsmith@example.com>"
are considered as a single author regardless of the different email addresses.

This change is just a workaround.

But as the URL for author details is constructed from Autor.dirname 
(as opposed to Author.email - see dc4d2a08) the original version does not work anyway.

This seems to me as a suitable quickfix until the whole issue is properly addressed.