style: Remove layout.css.webkit-line-clamp.enabled, and add a new pref to apply line-clamp to all BFCs

It's always true, so remove it.

Add another pref to allow -webkit-line-clamp to work on all blocks
rather than just legacy -webkit-boxes, which seems something we should
try to look into, eventually.

Depends on D155181

Differential Revision: https://phabricator.services.mozilla.com/D155182

components/style/properties/longhands/box.mako.rs

@@ -552,7 +552,6 @@ ${helpers.predefined_type(
     "LineClamp",
     "computed::LineClamp::none()",
     engines="gecko",
-    gecko_pref="layout.css.webkit-line-clamp.enabled",
     animation_value_type="ComputedValue",
     spec="https://drafts.csswg.org/css-overflow-3/#line-clamp",
 )}