Remove 'kind' field from span protocol (#15468)

Author: cleptric

develop-docs/sdk/telemetry/spans/span-protocol.mdx

@@ -53,7 +53,6 @@ The envelope item payload must contain an `items` array with span one and up to
       "name": "GET /users",
       "status": "ok",
       "is_segment": true,
-      "kind": "server",
       "start_timestamp": 1742921669.158209,
       "end_timestamp": 1742921669.180536,
       "attributes": {
@@ -115,7 +114,6 @@ The envelope item payload must contain an `items` array with span one and up to
       "name": "app.handle",
       "status": "ok",
       "is_segment": false,
-      "kind": "server",
       "start_timestamp": 1742921669.178306,
       "end_timestamp": 1742921669.180484,
       "attributes": {
@@ -149,7 +147,6 @@ The envelope item payload must contain an `items` array with span one and up to
 | `name` | string | Yes | A low cardinality description of what the span represents (e.g., "GET /users", "database.query") |
 | `status` | string | Yes | Status of the span operation. Either `"ok"` or `"error"` |
 | `is_segment` | boolean | Yes | Whether the span is a segment span |
-| `kind` | string | Yes | The kind of span. Values: `"server"`, `"client"`, `"producer"`, `"consumer"`, `"internal"` |
 | `start_timestamp` | number | Yes | Unix timestamp (with fractional microseconds) when the span was started |
 | `end_timestamp` | number | Yes | Unix timestamp (with fractional microseconds) when the span was ended |
 | `attributes` | object | No | Key-value pairs containing additional metadata about the span |