ref(starlette): Remove unused _serialize_request_body_data helper (#8146)

Removes `_serialize_request_body_data` from
`sentry_sdk/integrations/starlette.py`. Nothing calls it.

It was added in #6136 to serialize the request body for the segment span
under span streaming. #6286 changed the FastAPI path to serialize the
cached `request._json` / `request._form` directly and removed the only
call site (`_serialize_request_body_data(info["data"])`). The definition
stayed behind.

Why it's safe:
- The underscore-prefixed name doesn't appear anywhere else in the repo:
no references in `sentry_sdk/`, `tests/`, `scripts/`, or docs.
- None of the open PRs use it.
- The module still uses the `json` and `AnnotatedValue` imports
elsewhere, so this change doesn't remove any imports.
- Users see no change in behavior or public API.

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Co-authored-by: sentry-junior[bot] <264270552+sentry-junior[bot]@users.noreply.github.com>

Author: sentry-junior[bot]

sentry_sdk/integrations/starlette.py

@@ -277,16 +277,6 @@ async def _sentry_send(*args: "Any", **kwargs: "Any") -> "Any":
     return middleware_class
 
 
-def _serialize_request_body_data(data: "Any") -> str:
-    # data may be a JSON-serializable value, an AnnotatedValue, or a dict with AnnotatedValue values
-    def _default(value: "Any") -> "Any":
-        if isinstance(value, AnnotatedValue):
-            return value.value
-        return str(value)
-
-    return json.dumps(data, default=_default)
-
-
 @ensure_integration_enabled(StarletteIntegration)
 def _capture_exception(exception: BaseException, handled: "Any" = False) -> None:
     event, hint = event_from_exception(