Remove targetDesc helper in assign.c in favor of splitting cases for i18n

[MichaelChirico]

Gave up on the grammatical case-switching approach suggested here:

#6482 (comment)

In the end dropping targetDesc() altogether means about a dozen extra lines, I think the added clarity is worth it. As seen here, it also unconstrains us from shoehorning both branches into the same format, potentially adding clarity to the message.

[github-actions]

Generated via commit 9266548

Download link for the artifact containing the test results: ↓ atime-results.zip

Time taken to finish the standard R installation steps: 12 minutes and 6 seconds

Time taken to run atime::atime_pkg on the tests: 7 minutes and 1 seconds

[tdhock]

looks fine. i wonder if it is worth adding a comment about the history with targetDesc? (ok if you dont think it is necessary)

[MichaelChirico]

adding a comment about the history with targetDesc

In the code somewhere? (as a comment on the line where it used to be declared?) or here in the PR?

[MichaelChirico]

I don't think a code comment is needed (it was not a particularly important part of the code, no "historic" relevance, and git is here for future archeologists), and we can edit the PR if needed later; submitting now.