chore(*): Remove unnecessary inherit that overrides selection in pin area.

Author: MKirova

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

@@ -1948,7 +1948,6 @@
 
     %grid-cell--pinned {
         position: relative;
-        background: inherit;
         z-index: 9999;
     }