Remove dependency on taxize package if it is orphaned

[dlebauer]

https://github.com/ropensci/taxize/issues/936

If the taxize package is orphaned:

Message from CRAN:

Package 'taxize' required the archived (and oft-failing) package 'bold', and a request to change that was ignored. We have fixed 'taxize' but orphaned it, so your package now shows a warning about requiring an orphaned package (which is not allowed by the CRAN policy). Please remove the requirement or change to Suggests and use conditionally.

Steps to resolve

• [ ] Identify and remove uses of taxize from code & DESCRIPTION
• [ ] Update docs
• [ ] Increment package version
• [ ] R CMD check
• [ ] Update NEWS
• [ ] Submit to CRAN