Remove dependency on taxize package if it is orphaned

[dlebauer]

ropensci/taxize#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