Build architecture-indepenent packages again (release branch)

[MR 1/2 to fix this in release packages, has to be tagged for releasing a new revision after merging]

There are no architecture-specific binaries in this package, so it should build an architecture-independent package (arch=all) rather than architecture-dependent packages for any architecture (arch=any).

The original change was done in the context of some MR regarding Raspbian, but this also doesn't make sense there. It looks like this was done as a workaround to fix the following error, which was properly fixed by setting ICINGA_BUILD_DEB_DEFAULT_ARCH in 19f52aec:

dpkg-genbuildinfo: error: binary build with no binary artifacts found; .buildinfo is meaningless

This reverts commit 31b75467.