Remove CSS2 backgrounds tests from content_shell.filter

The following tests run on headless shell, so can be removed from
content_shell.filter

- external/wpt/css/CSS2/backgrounds/background-applies-to-006.xht
- external/wpt/css/CSS2/backgrounds/background-applies-to-012.xht
- external/wpt/css/CSS2/backgrounds/background-color-applies-to-006.xht
- external/wpt/css/CSS2/backgrounds/background-color-applies-to-012.xht
- external/wpt/css/CSS2/backgrounds/background-image-applies-to-006.xht
- external/wpt/css/CSS2/backgrounds/background-image-applies-to-012.xht
- external/wpt/css/CSS2/backgrounds/background-position-applies-to-012.xht
- external/wpt/css/CSS2/backgrounds/background-repeat-applies-to-006.xht
- external/wpt/css/CSS2/backgrounds/background-repeat-applies-to-012.xht

Change-Id: I72a3a5ec62e13ef9acbd0c5fe0f8d84b9456e6f2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7768379
Reviewed-by: Koji Ishii <kojii@chromium.org>
Reviewed-by: Christian Biesinger <cbiesinger@chromium.org>
Commit-Queue: Sharon Yang <yangsharon@igalia.com>
Cr-Commit-Position: refs/heads/main@{#1626507}
diff --git a/third_party/blink/web_tests/TestLists/content_shell.filter b/third_party/blink/web_tests/TestLists/content_shell.filter
index 84de29e..41c54749 100644
--- a/third_party/blink/web_tests/TestLists/content_shell.filter
+++ b/third_party/blink/web_tests/TestLists/content_shell.filter
@@ -63,15 +63,6 @@
 external/wpt/css/compositing/mix-blend-mode/mix-blend-mode-with-transform-and-preserve-3D.html
 external/wpt/css/compositing/root-element-opacity-change.html
 external/wpt/css/compositing/svg/mix-blend-mode-svg-rectangle.html
-external/wpt/css/CSS2/backgrounds/background-applies-to-006.xht
-external/wpt/css/CSS2/backgrounds/background-applies-to-012.xht
-external/wpt/css/CSS2/backgrounds/background-color-applies-to-006.xht
-external/wpt/css/CSS2/backgrounds/background-color-applies-to-012.xht
-external/wpt/css/CSS2/backgrounds/background-image-applies-to-006.xht
-external/wpt/css/CSS2/backgrounds/background-image-applies-to-012.xht
-external/wpt/css/CSS2/backgrounds/background-position-applies-to-012.xht
-external/wpt/css/CSS2/backgrounds/background-repeat-applies-to-006.xht
-external/wpt/css/CSS2/backgrounds/background-repeat-applies-to-012.xht
 external/wpt/css/CSS2/borders/border-applies-to-006.xht
 external/wpt/css/CSS2/borders/border-applies-to-012.xht
 external/wpt/css/CSS2/borders/border-color-applies-to-006.xht