Remove shadowrootmode demo

[pepelsbey]

Description

Reverting mdn/interactive-examples/pull/2707 since I moved demos to mdn/dom-examples/pull/245

Motivation

To support existing implementations in Chrome and Safari and for the upcoming one in Firefox 122 (behind the flag).

Related issues and pull requests

• https://github.com/mdn/content/pull/31577
• https://github.com/mdn/content/pull/31490