Remove unused code

These files contain code whose last callers were removed two years ago
with https://crrev.com/c/2360314.

Bug: 846357
Change-Id: I04279862f55abfbdc74f7ffc37822311d8f59b9e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4053302
Auto-Submit: Avi Drissman <avi@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Commit-Queue: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1075682}

third_party/blink/renderer/platform/BUILD.gn

@@ -1598,8 +1598,6 @@ component("platform") {
       "mac/color_mac.mm",
       "mac/graphics_context_canvas.h",
       "mac/graphics_context_canvas.mm",
-      "mac/web_core_ns_cell_extras.h",
-      "mac/web_core_ns_cell_extras.mm",
       "text/locale_mac.h",
       "text/locale_mac.mm",
       "theme/web_theme_engine_mac.cc",