Simplify oauth (#8972)

* Remove unused OauthProvider::GetIdentifier

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>

* Reuse gitlab's getAuthData() instead of open-coding it

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>

* Remove OauthProvider::GetAuthDataFromJson interface

The data is already available via GetUserFromJson().AuthData

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>

einterfaces/oauthproviders.go

@@ -10,9 +10,7 @@ import (
 )
 
 type OauthProvider interface {
-	GetIdentifier() string
 	GetUserFromJson(data io.Reader) *model.User
-	GetAuthDataFromJson(data io.Reader) string
 }
 
 var oauthProviders = make(map[string]OauthProvider)