style: Remove LengthPercentage::was_calc.

There should not be any behavior change between specifying a percentage using %
or calc(%) per the resolution of https://github.com/w3c/csswg-drafts/issues/3482.

Differential Revision: https://phabricator.services.mozilla.com/D43747

components/style/values/specified/position.rs

@@ -301,7 +301,6 @@ impl<S: Side> ToComputedValue for PositionComponent<S> {
                     l,
                     Some(p),
                     length.clamping_mode,
-                    /* was_calc = */ true,
                 )
             },
             PositionComponent::Side(_, Some(ref length)) |