test: remove podman-plugins workaround for RHEL/CentOS

https://bugzilla.redhat.com/show_bug.cgi?id=1877865 is now fixed and
podman-plugins is now shipped in stable RHEL. One less hack in our codebase!

Signed-off-by: Ondřej Budai <ondrej@budai.cz>

test/data/vendor/87-podman-bridge.conflist

@@ -1,41 +0,0 @@
-{
-  "cniVersion": "0.4.0",
-  "name": "podman",
-  "plugins": [
-    {
-      "type": "bridge",
-      "bridge": "cni-podman0",
-      "isGateway": true,
-      "ipMasq": true,
-      "hairpinMode": true,
-      "ipam": {
-        "type": "host-local",
-        "routes": [{ "dst": "0.0.0.0/0" }],
-        "ranges": [
-          [
-            {
-              "subnet": "10.88.0.0/16",
-              "gateway": "10.88.0.1"
-            }
-          ]
-        ]
-      }
-    },
-    {
-      "type": "portmap",
-      "capabilities": {
-        "portMappings": true
-      }
-    },
-    {
-      "type": "firewall"
-    },
-    {
-      "type": "tuning"
-    },
-    {
-      "domainName": "dns.podman",
-      "type": "dnsname"
-    }
-  ]
-}

test/data/vendor/README.md

@@ -1,9 +0,0 @@
-This directory includes 3rd party modules, needed in CI.
-
- - [`dnsname`](https://github.com/containers/dnsname) plugin for podman,
-   needed to translate host names of containers into IPs. It is shipped
-   in Fedora, but missing in RHEL 8, see
-   [rhgbz#1877865](https://bugzilla.redhat.com/show_bug.cgi?id=1877865).
-   The `87-podman-bridge.conflist` file contains the corresponding config,
-   where the `{"domainName": "dns.podman", "type": "dnsname"}` bit is the
-   newly added part.