Remove enable-iframe-based-signin flag.

components/signin/core/common/signin_switches.cc

Index: components/signin/core/common/signin_switches.cc
diff --git a/components/signin/core/common/signin_switches.cc b/components/signin/core/common/signin_switches.cc
index 88647c86f2e8f9bb43a1fd7176478f77dbbf443e..9b97b463c950b8660fff9a04061e0abc8ceb069f 100644
--- a/components/signin/core/common/signin_switches.cc
+++ b/components/signin/core/common/signin_switches.cc
@@ -26,10 +26,6 @@ const char kDisableSigninScopedDeviceId[] = "disable-signin-scoped-device-id";
 // Enables consistent identity features.
 const char kEnableAccountConsistency[] = "enable-account-consistency";
 
-// Enables the old iframe-based flow for sign in.  When not enabled, uses the
-// webview-based flow.
-const char kEnableIframeBasedSignin[] = "enable-iframe-based-signin";
-
 // Enables new gaia password-separated sign in flow.
 const char kEnablePasswordSeparatedSigninFlow[] =
     "enable-password-separated-signin-flow";