Restore the HTML form on DDG when using safest in 128

When rebasing to 128, I had to drop our patch that sent DDG searches through the simplified HTML because of 66c7c878.

This commit modifies toolkit/components/search/SearchEngine.sys.mjs so that only the charset is passed to the function we used to modify instead of the full engine object.

Having the full object was useful for us, because it's how we used to tell it was DDG.

The workaround was not trivial, so I dropped the patch and I'm opening this MR to re-implement it.