[R-pkg-team] Bug#976802: r-cran-sf build depends on r-cran-s2 that is only available on amd64/ppc64/ppc64el/x32

[Adrian Bunk]

Source: r-cran-sf
Version: 0.9-6+dfsg-1
Severity: serious
Tags: ftbfs

r-cran-sf build depends on r-cran-s2 that is only available
on amd64/ppc64/ppc64el/x32 (see #976473).

Since the new r-cran-sf build dependency seems to be only
required for tests, removing it (also from the autopkgtest?)
might be the easiest short-term solution.