minor magmaR update for compatibility with upcoming vcr v2.0.0 release

[dtm2451]

PR removes a call to a function that is being deprecated in the next version of vcr, due to it no longer being able to be made fully accurate.

Notably:

• This has no functional impact on the package outside of ours and CRAN's testing -- vcr is only used in unit testing. (Further, we weren't using vcr::check_cassette_names()'s intended purpose anyway. I believe I only included the call to the function because it was a suggestion in the package's previous vignette!)
• I'm not sure if we even need to bother bumping the version and re-submitting to CRAN in the short term because function is only being deprecated. It will still be kept in the package for a while, just with a note thrown that shouldn't matter to CRAN. I'll be notified by CRAN when it gets actually removed and magmaR starts failing tests, and we can await having real updates to mix in to our next version until then!