Remove CXX_STD=CXX11 #135

Job	Run time
R-CMD-check / setup	8s
website / pkgdown	2m 25s
routine / routine	2m 47s
R-CMD-check / macOS-latest (4.4.1)	2m 33s
R-CMD-check / windows-latest (devel)	5m 42s
R-CMD-check / windows-latest (4.4.1)	8m 27s
R-CMD-check / windows-latest (4.2)	10m 16s
R-CMD-check / windows-latest (4.1)	9m 49s
R-CMD-check / windows-latest (3.6)	2m 17s
R-CMD-check / ubuntu-20.04 (devel)	4m 10s
R-CMD-check / ubuntu-20.04 (4.4.1)	4m 16s
R-CMD-check / ubuntu-20.04 (4.3.3)	4m 9s
R-CMD-check / ubuntu-20.04 (4.2.3)	4m 48s
R-CMD-check / ubuntu-20.04 (4.1.3)	4m 28s
R-CMD-check / ubuntu-20.04 (4.0.5)	4m 37s
	1h 10m 52s

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