Remove recommendation to use deprecated fetchSignInMethods (#12148)

Author: renkelvin

FirebaseAuth/Sources/Public/FirebaseAuth/FIRUser.h

@@ -261,11 +261,10 @@ NS_SWIFT_NAME(User)
             Auth section of the Firebase console.
         + `AuthErrorCodeEmailAlreadyInUse` -  Indicates the email asserted by the credential
             (e.g. the email in a Facebook access token) is already in use by an existing account,
-            that cannot be authenticated with this method. Call `Auth.fetchSignInMethods(forEmail:)`
-            for this user’s email and then prompt them to sign in with any of the sign-in providers
-            returned. This error will only be thrown if the "One account per email address"
-            setting is enabled in the Firebase console, under Auth settings. Please note that the
-            error code raised in this specific situation may not be the same on Web and Android.
+            that cannot be authenticated with this method. This error will only be thrown if the
+          "One account per email address" setting is enabled in the Firebase console, under Auth
+          settings. Please note that the error code raised in this specific situation may not be
+          the same on Web and Android.
         + `AuthErrorCodeUserDisabled` - Indicates the user's account is disabled.
         + `AuthErrorCodeWrongPassword` - Indicates the user attempted reauthentication with
             an incorrect password, if credential is of the type `EmailPasswordAuthCredential`.