vault: drop the injector's anti-affinity

The chart requires injector pods on different nodes. With one node and one replica the rolling update to vault-k8s 1.7.6 deadlocked: the new pod stayed Pending until the old one left, which waits for the new one. The server already sets affinity to "" for the same reason.

vault/overrides.yaml

@@ -3,6 +3,10 @@ global:
    tlsDisable: true
 injector:
    enabled: true
+   # The chart's default anti-affinity forbids two injector pods on one
+   # node, so on this single node a rolling update can never start the new
+   # pod while the old one runs.
+   affinity: ""
 server:
    # Pinned to the running version: the chart would otherwise pull its own
    # default on the next pod restart, and with Shamir unsealing a Vault