Remove SET_TYPEOF() call

[MichaelChirico]

Part of #6180 thanks to the helpful suggestion in R-exts:

An alternative way to construct the expression that will work in
any @R{} version is

@example
SEXP expr = LCONS(R_NilValue, allocList(2));
@end example

[github-actions]

Generated via commit 57cb810

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

Time taken to finish the standard R installation steps: 11 minutes and 50 seconds

Time taken to run atime::atime_pkg on the tests: 3 minutes and 35 seconds

[ben-schwen]

LGTM