style: Drop layout.css.background-blend-mode.enabled pref.

It was enabled by default in bug 970600.

Differential Revision: https://phabricator.services.mozilla.com/D15708

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

@@ -108,7 +108,6 @@ ${helpers.single_keyword(
     color-burn hard-light soft-light difference exclusion hue
     saturation color luminosity""",
     gecko_constant_prefix="NS_STYLE_BLEND",
-    gecko_pref="layout.css.background-blend-mode.enabled",
     vector=True, products="gecko", animation_value_type="discrete",
     spec="https://drafts.fxtf.org/compositing/#background-blend-mode",
     flags="APPLIES_TO_FIRST_LETTER APPLIES_TO_FIRST_LINE APPLIES_TO_PLACEHOLDER",