style: Remove obsolete comment to add use counter for developing CSS properties

The CSS properties in the list are all shipped. Since we still don't support use
counter for developing CSS properties since this bug opens, I assume this
feature is not critical. Hence the removal of the comment.

Differential Revision: https://phabricator.services.mozilla.com/D180771

components/style/properties/counted_unknown_properties.py

@@ -2,16 +2,6 @@
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
-# The following properties are under development, so they are not in this list.
-# FIXME: We should handle the developing properties properly by Bug 1577358:
-#    "backdrop-filter",
-#    "text-decoration-skip-ink",
-#    "column-span",
-#    "offset-distance",
-#    "offset-path",
-#    "offset-rotate",
-#    "offset",
-#    "text-underline-position",
 COUNTED_UNKNOWN_PROPERTIES = [
     "-webkit-font-smoothing",
     "-webkit-tap-highlight-color",