Drop gettext and use stopf directly

[MichaelChirico]

Minor change for consistency.

This could have been part of #7260 -- there, gettext() was used since the message would be passed to either stop() or warning(); now, it's always passed to stop(), so we don't need to separate into two steps.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.97%. Comparing base (e26cfb3) to head (ffc0925).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7528      +/-   ##
==========================================
- Coverage   98.97%   98.97%   -0.01%     
==========================================
  Files          87       87              
  Lines       16739    16738       -1     
==========================================
- Hits        16567    16566       -1     
  Misses        172      172              

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[github-actions]

No obvious timing issues in HEAD=rm-gettext

Generated via commit ffc0925

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

Task	Duration
R setup and installing dependencies	4 minutes and 42 seconds
Installing different package versions	10 minutes and 46 seconds
Running and plotting the test cases	3 minutes and 25 seconds

[ben-schwen]

TY!