Fix undeclared `stringr` reference in `xrefs`

[stufield]

SeqId has a reference to stringr in the refs, but it's not a dependency. Should remove this reference or make stringr a Suggests.

• source: https://cran.r-project.org/web/checks/check_results_SomaDataIO.html
• actually not sure will do this one as the link in the pkgdown actually do follow to stringr as is.
• perhaps will need to add stringr to Suggests, though not my preference

[stufield]

It looks like the xrefs is just a warning, the links do work in the pkgdown.
Closing.