chore: remove superseded schema files

Remove schema/commands/ and schema/message.json, replaced by
the new schema/{c2s,s2c,s2s}/*.schema.json structure.

schema/commands/JOIN.json

@@ -1,23 +0,0 @@
-{
-  "$schema": "https://json-schema.org/draft/2020-12/schema",
-  "$id": "https://git.eeqj.de/sneak/chat/schema/commands/JOIN.json",
-  "title": "JOIN",
-  "description": "Join a channel. C2S: request to join. S2C: notification that a user joined. RFC 1459 §4.2.1.",
-  "$ref": "../message.json",
-  "properties": {
-    "command": { "const": "JOIN" },
-    "from": {
-      "type": "string",
-      "description": "Nick that joined (S2C only)."
-    },
-    "to": {
-      "type": "string",
-      "description": "Channel name.",
-      "pattern": "^#[a-zA-Z0-9_-]+$"
-    }
-  },
-  "required": ["command", "to"],
-  "examples": [
-    { "command": "JOIN", "from": "alice", "to": "#general" }
-  ]
-}