Remove obsolete workaround for integer64 case

[MichaelChirico]

Closes #6480. I assume existing tests are enough -- R/groupingsets.R has 100% coverage:

https://app.codecov.io/gh/Rdatatable/data.table/blob/master/R%2Fgroupingsets.R

No NEWS since this is purely internal.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.02%. Comparing base (4d30c7f) to head (1a81a2e).
⚠️ Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7576      +/-   ##
==========================================
- Coverage   99.02%   99.02%   -0.01%     
==========================================
  Files          87       87              
  Lines       16750    16742       -8     
==========================================
- Hits        16587    16579       -8     
  Misses        163      163              

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[github-actions]

• HEAD=bit64-workaroud slower P<0.001 for DT[by,verbose=TRUE] improved in #6296
  

Generated via commit 1a81a2e

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

Task	Duration
R setup and installing dependencies	5 minutes and 4 seconds
Installing different package versions	11 minutes and 30 seconds
Running and plotting the test cases	4 minutes and 26 seconds

[ben-schwen]

LGTM.

Test 2083.2 covers the INT64 NA rbindlist case.