ref(node): Remove vendored LangChain instrumentation

This instrumentation is dead code: the LangChain integration is provided by the
channel-based implementation in `@sentry/server-utils` (default since #22501),
and the vendored OpenTelemetry instrumentation here is no longer wired up.

Ref: #22346

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: andreiborza