bridge, spoof check: remove drop rule index

Rules are appendend by default, thus using an index is redundant.
Using an index also requires the full NFT cache, which causes a CNI ADD
to be extremely slow.

Signed-off-by: Miguel Duarte Barroso <mdbarroso@redhat.com>

pkg/link/spoofcheck_test.go

@@ -254,7 +254,6 @@ func assertExpectedRulesInSetupConfig(c configurerStub) {
                     "comment":"macspoofchk-container99-net1"}},
                 {"rule":{"family":"bridge","table":"nat","chain":"cni-br-iface-container99-net1-mac",
                     "expr":[{"drop":null}],
-                    "index":0,
                     "comment":"macspoofchk-container99-net1"}}
             ]}`
 	ExpectWithOffset(1, string(jsonConfig)).To(MatchJSON(expectedConfig))