[Explainer] Remove web bundle directFromSellerSignals (#1368)

* [Explainer] Remove web bundle directFromSellerSignals

Header-based directFromSellerSignals (directFromSellerSignalsHeaderAdSlot), which replaces web bundle directFromSellerSignals, is unaffected.

Usage of web bundle directFromSellerSignals is extremely low: https://chromestatus.com/metrics/feature/timeline/popularity/5034.

The feature was originally requested in #119, and the header-based version was requested in #119 (comment).

Blink I2D&R thread: https://groups.google.com/a/chromium.org/g/blink-dev/c/t9gHiTwV370

Chromium removal bug: https://crbug.com/384481095

* Update FLEDGE.md

Fix broken links.