fix(grid): update loading outlet styles to remove modal background

Author: dkamburov

projects/igniteui-angular/core/src/core/styles/components/grid/_grid-theme.scss

@@ -2374,6 +2374,12 @@
         --ig-size: var(--grid-size);
     }
 
+    .igx-grid__loading-outlet {
+        .igx-overlay__wrapper--modal {
+            background: none;
+        }
+    }
+
     %igx-grid__addrow-snackbar {
         position: absolute;
         z-index: 5;