style: Remove pointless assertion.

It's a reference, it's not going to be null, and if it was, it would've already
been UB on the caller.

components/style/gecko_bindings/sugar/ownership.rs

@@ -109,7 +109,6 @@ pub unsafe trait HasArcFFI : HasFFI {
     ///
     /// &GeckoType -> &Arc<ServoType>
     fn as_arc<'a>(ptr: &'a &Self::FFIType) -> &'a RawOffsetArc<Self> {
-        debug_assert!(!(ptr as *const _).is_null());
         unsafe {
             transmute::<&&Self::FFIType, &RawOffsetArc<Self>>(ptr)
         }