Don't block autopkgtest on uninstallable archs

Currently when a package is uninstallable on an arch, no autopkgtests for that arch are triggered
and the autopkgtest policy blocks migration. However it's not the job of the autopkgtest policy
to judge uninstallability and packages that build an arch:all package that just isn't installable
on the autopkgtest arch should not be blocked for this.

Closes: #918620

britney2/policies/autopkgtest.py

@@ -363,9 +363,8 @@ class AutopkgtestPolicy(BasePolicy):
                     self.logger.info('%s hasn''t been built on arch %s, delay autopkgtest there', source_name, arch)
                     excuse.add_verdict_info(verdict, "arch:%s not built yet, autopkgtest delayed there" % arch)
                 elif arch in excuse.unsatisfiable_on_archs:
-                    verdict = PolicyVerdict.REJECTED_TEMPORARILY
-                    self.logger.info('%s is uninstallable on arch %s, delay autopkgtest there', source_name, arch)
-                    excuse.add_verdict_info(verdict, "uninstallable on arch %s, autopkgtest delayed there" % arch)
+                    self.logger.info('%s is uninstallable on arch %s, not running autopkgtest there', source_name, arch)
+                    excuse.addinfo("uninstallable on arch %s, not running autopkgtest there" % arch)
                 else:
                     self.request_tests_for_source(item, arch, source_data_srcdist, pkg_arch_result)