packit: Drop targets list from COPR build

Hardcoding this list is redundant and hard to change: changing the COPR
config and updating all our projects' packit.yml files needs to happen
in lockstep.

packit now learned to just read the configuration from an existing COPR
repository, so drop the target list entirely. See
packit/packit-service#1499

packit.yaml

@@ -28,11 +28,6 @@ jobs:
       owner: "@cockpit"
       project: "cockpit-preview"
       preserve_project: True
-      targets:
-      - fedora-36
-      - fedora-37
-      - centos-stream-8-x86_64
-      - centos-stream-9-x86_64
     actions:
       post-upstream-clone: make cockpit-ostree.spec
       # HACK: tarball for releases (copr_build, koji, etc.), copying spec's Source0; this