Remove unused css variable

Author: hoonji

tensorboard/webapp/runs/views/runs_table/runs_data_table.scss

@@ -15,7 +15,6 @@ limitations under the License.
 @use '@angular/material' as mat;
 @import 'tensorboard/webapp/theme/tb_theme';
 $_circle-size: 20px;
-$_arrow_size: 16px;
 
 :host {
   min-width: 100%;