Remove user-facing references to .internal.selfref

[MichaelChirico]

Closes #4519. Supersedes #6696.

Usage in internal errors is retained:

data.table/src/assign.c

Line 125 in 70c64ac

if (!isNull(p)) internal_error(__func__, ".internal.selfref ptr is neither NULL nor R_NilValue"); // # nocov

data.table/src/assign.c

Line 127 in 70c64ac

if (!(isNull(tag) || isString(tag))) internal_error(__func__, ".internal.selfref tag is neither NULL nor a character vector"); // # nocov

cc @aitap and @spiddy1204

[github-actions]

Generated via commit 8e4783d

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

Task	Duration
R setup and installing dependencies	4 minutes and 27 seconds
Installing different package versions	8 minutes and 5 seconds
Running and plotting the test cases	2 minutes and 31 seconds

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.62%. Comparing base (70c64ac) to head (8e4783d).
Report is 12 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6698   +/-   ##
=======================================
  Coverage   98.61%   98.62%           
=======================================
  Files          79       79           
  Lines       14553    14630   +77     
=======================================
+ Hits        14352    14429   +77     
  Misses        201      201           

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