[M-36899] Remove deprecated SourcePluginId (#17884)

app/plugin_requests.go

@@ -71,7 +71,6 @@ func (a *App) ServeInterPluginRequest(w http.ResponseWriter, r *http.Request, so
 	context := &plugin.Context{
 		RequestId: model.NewId(),
 		UserAgent: r.UserAgent(),
-		SourcePluginId: sourcePluginId,
 	}
 
 	r.Header.Set("Mattermost-Plugin-ID", sourcePluginId)

plugin/context.go

@@ -12,5 +12,4 @@ type Context struct {
 	IpAddress      string
 	AcceptLanguage string
 	UserAgent      string
-	SourcePluginId string // Deprecated: Use the "Mattermost-Plugin-ID" HTTP header instead
 }