remove unnecessary omp single that cause deadlock (fixes #6273) #8223

Job	Run time
r-package (debian, R-devel, clang-16)	16m 26s
r-package (ubuntu-latest, gcc, R 3.6, cmake)	8m 8s
r-package (debian, R-devel, clang-17)	16m 18s
r-package (ubuntu-latest, gcc, R 4.3, cmake)	9m 20s
r-package (ubuntu-latest, clang, R 3.6, cmake)	8m 56s
r-sanitizers (ubuntu-latest, R-devel, gcc ASAN/UBSAN)	15m 6s
r-package (ubuntu-latest, clang, R 4.3, cmake)	9m 59s
r-sanitizers (ubuntu-latest, R-devel, clang ASAN/UBSAN)	28m 22s
r-package (macOS-latest, gcc, R 4.3, cmake)	24m 16s
r-package (macOS-latest, clang, R 4.3, cmake)	12m 35s
r-package (windows-latest, MINGW, R 3.6, cmake)	14m 10s
r-package (windows-latest, MINGW, R 4.3, cmake)	17m 15s
r-package (windows-2019, MSVC, R 3.6, cmake)	9m 56s
r-package (windows-2022, MSVC, R 4.3, cmake)	29m 4s
r-package (windows-latest, MINGW, R 3.6, cran)	19m 35s
r-package (windows-latest, MINGW, R 4.3, cran)	19m 40s
r-package (ubuntu-latest, gcc, R 4.3, cran)	15m 35s
r-package (macOS-latest, clang, R 4.3, cran)	23m 14s
r-rchk (ubuntu-latest, gcc, R 4.3, cran)	10m 24s
all-r-package-jobs-successful	2s
	5h 8m 21s

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