style: scroll-margin properties shouldn't apply to first-line / first-letter.

From https://drafts.csswg.org/css-scroll-snap-1/#placement:

> None of the properties in this module apply to the ::first-line and
> ::first-letter pseudo-elements.

Differential Revision: https://phabricator.services.mozilla.com/D37719

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

@@ -38,7 +38,6 @@
         logical=side[1],
         logical_group="scroll-margin",
         spec="https://drafts.csswg.org/css-scroll-snap-1/#propdef-scroll-margin-%s" % side[0],
-        flags="APPLIES_TO_FIRST_LETTER APPLIES_TO_FIRST_LINE",
         animation_value_type="ComputedValue",
     )}
 % endfor