ref(node): Remove vendored Firebase instrumentation (#22732)

## What

Removes the dead vendored OpenTelemetry Firebase instrumentation from
`@sentry/node`.

## Why

The Firebase integration is now provided by the channel-based
implementation in `@sentry/server-utils` (the default since #22501). The
vendored instrumentation here is no longer wired into the SDK, so it is
pure dead code.

Ref: #22346

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: andreiborza