Revert of Content "gn check" work.

content/public/plugin/BUILD.gn

Index: content/public/plugin/BUILD.gn
diff --git a/content/public/plugin/BUILD.gn b/content/public/plugin/BUILD.gn
index 1a0bfd920ffac4c2e06e3915ad1b185f61455a97..e4c4d3681933cc26923da160a63f76c471202bf9 100644
--- a/content/public/plugin/BUILD.gn
+++ b/content/public/plugin/BUILD.gn
@@ -24,13 +24,7 @@
 
   deps = [
     "//base",
-    "//content/plugin",  # Must not be a public dep.
+    "//content/plugin",
     "//content/public/common:common_sources",
   ]
-
-  allow_circular_includes_from = [
-    # This target is a pair with content/plugin. They always go together and
-    # include headers from each other.
-    "//content/plugin",
-  ]
 }