Remove CSSStickyPosition runtime flag

position:sticky shipped always-on in M56.

Bug: 231752
Change-Id: I7bff1d34540745a7220358965a933141d6f172e7
Reviewed-on: https://chromium-review.googlesource.com/896143Reviewed-by: Robert Flack <flackr@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Commit-Queue: Stephen McGruer <smcgruer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533688}

third_party/WebKit/Source/platform/runtime_enabled_features.json5

@@ -290,10 +290,6 @@
       name: "CSSSnapSize",
       status: "experimental",
     },
-    {
-      name: "CSSStickyPosition",
-      status: "stable",
-    },
     {
       name: "CSSTransformBox",
       status: "stable",