d/rules: filter the content of the d/*.substvars files

This fixes some dpkg-gencontrol warnings:
dpkg-gencontrol: warning: package diffoscope: substitution variable ${diffoscope-minimal:Recommends} unused, but is defined
dpkg-gencontrol: warning: package diffoscope-minimal: substitution variable ${diffoscope:Recommends} unused, but is defined

Signed-off-by: Mattia Rizzolo <mattia@debian.org>