No. In ArcGIS Dashboards, it is not possible to configure the Layers list to remove an operational layer. The Layers list displays all operational layers on web maps added to dashboards. When viewing a dashboard, the layer visibility of an operational layer can only be turned on or off by clicking the eye symbol on the list. This map interaction does not change the layers in the dashboard, and upon refreshing, each operational layer displays according to the default layer visibility set in the web map.
To remove an operational layer from the Layers list in ArcGIS Dashboards, remove the layer from the web map.
Alternatively, to remove a layer in ArcGIS Dashboards while keeping it in a web map, create a web app using ArcGIS Web AppBuilder and configure the Layer List widget in the web app. For instructions on how to select layers to display on the list in web apps, refer to ArcGIS Web AppBuilder: Configure the Layer List widget.
Use the following steps to add a web app to a dashboard as embedded content using the web app URL.