Remove outdated comment from content_settings_client.h

The removed comment speaks about StubContentSettingsClient, which was not created in https://codereview.chromium.org/646973002 after all, and thus does not exist in the ToT.

This is a comment-only change, thus skipping tests.

TBR=blundell@chromium.org
NOTRY=true
BUG=393248

Review URL: https://codereview.chromium.org/663933003

Cr-Commit-Position: refs/heads/master@{#301077}

components/content_settings/core/browser/content_settings_client.h

@@ -21,9 +21,6 @@ class CookieOptions;
 namespace content_settings {
 
 // An abstraction of operations that depend on the embedder (e.g. Chrome).
-// For mock / testing implementation in tests you can inherit from
-// StubContentSettingsClient instead to avoid the need to stub unneeded pure
-// virtual methods.
 class ContentSettingsClient {
  public:
   enum AccessType { BLOCKED, ALLOWED };