Dashboard: Reverted the changes of hiding multi-select and all variable in the datasource picker (#53521)

* REVERT PanelEdit: Hide multi-/all-select datasource variables in datasource picker, PR 52142

* user essentials mob! 🔱

lastFile:public/app/features/dashboard/dashgrid/PanelChrome.tsx

* user essentials mob! 🔱

lastFile:public/app/features/dashboard/dashgrid/PanelChrome.tsx

* removed todo comments

Co-authored-by: polinaboneva <polina.boneva@grafana.com>
Co-authored-by: Joao Silva <joao.silva@grafana.com>
Co-authored-by: eledobleefe <laura.fernandez@grafana.com>

public/app/features/variables/guard.ts

@@ -25,7 +25,6 @@ import {
   VariableQueryEditorProps,
   VariableWithMultiSupport,
   VariableWithOptions,
-  DataSourceVariableModel,
 } from './types';
 
 /** @deprecated use a if (model.type === "query") type narrowing check instead */
@@ -43,10 +42,6 @@ export const isConstant = (model: VariableModel): model is ConstantVariableModel
   return model.type === 'constant';
 };
 
-export const isDataSource = (model: VariableModel): model is DataSourceVariableModel => {
-  return model.type === 'datasource';
-};
-
 export const isMulti = (model: VariableModel): model is VariableWithMultiSupport => {
   const withMulti = model as VariableWithMultiSupport;
   return withMulti.hasOwnProperty('multi') && typeof withMulti.multi === 'boolean';