Depend on debhelper >= 10 and drop dh-systemd dependency (Closes: #958626)

Depend on debhelper >= 10 and drop dh-systemd dependency (Closes: #958626)

dh-systemd was merged into debhelper in version 9.20160709 [1] and since
stretch, dh-systemd is an empty transitional package.

For bullseye there's an intend to drop this empty transitional package.

This commit drops the "| dh-systemd" alternative Build-Depends as it is no
longer required (not even for backports) and is only confusing.

For Debian Jessie, it's easier to add backported debhelper >= 10 to the
repository.