Remove legacy session replay file attachment in favor of Session Replay envelope

[tustanivsky]

This PR removes the legacy path that uploaded the session replay clip as a raw .mp4 file attachment on the crash event, now that the Session Replay product integration is fully functional across platforms.

Both paths were shipping the same recording in two different forms:

• Native platforms (Windows/Linux/Mac): the plugin attached the staged .mp4 in the on_crash hook (GenericPlatformSentrySubsystem::OnCrash), while the native SDK's replay envelope pipeline (sentry__session_replay_flush_pending) also consumed the same clip.
• Cocoa (macOS/iOS): AppleSentrySubsystem both called UploadSessionReplayForEvent (file attachment) and FAppleSentryReplayEnvelope::CaptureForCrashEvent (envelope).

Beyond being redundant, the two mechanisms actively conflicted on the crashpad backend - the envelope pipeline consumes and deletes the staged replay-<id>.mp4 while assembling the crash report. Since crashpad copies file attachments out-of-process at dump time (after the in-process on_crash hook and after the envelope flush already deleted the file) the file attachment was silently dropped (attachment couldn't be opened, skipping). So on crashpad the legacy path didn't even work; the envelope is the reliable delivery path.

Documentation:

• docs(unreal): Remove legacy Session Replay attachment mentions sentry-docs#18810

Related items:

• Add integration with Session Replay product for native platforms #1445
• Add integration with Session Replay product for Apple platforms #1468