samba: remove the unimplemented globalConfig value

samba.globalConfig has been part of values.yaml since the chart's first
commit but is read by no template. Setting it never had an effect.

Rather than implement it, the key is removed and the strict schema now
rejects it: additionalProperties is already false on the samba object, so
dropping the property turns a silently ignored key into a render-time
error.

Operationally inert: neither the cluster-smb fleet bundle nor the deployed
release's user-supplied values set the key -- it only ever appeared as the
chart's own null default.

Major, because a published value disappears and is now rejected.

Fixes #117

samba/values.schema.json

@@ -82,10 +82,6 @@
         "securityContext": {
           "$ref": "#/definitions/securityContext"
         },
-        "globalConfig": {
-          "description": "Currently read by no template - see the note in the PR that introduced this schema. Kept allowed because it is part of the published values surface; whether it gets implemented or removed is a separate decision.",
-          "type": ["string", "null"]
-        },
         "storageClass": {
           "type": ["string", "null"]
         }