Revert "fix(button-group): remove unnecessary bootstrap-specific styles (#16259)" (#16261)

This reverts commit 37a33f6.

Author: simeonoff

projects/igniteui-angular/src/lib/core/styles/components/button-group/_button-group-theme.scss

@@ -65,6 +65,11 @@
         &[igxButton] {
             border-radius: 0;
             border-color: var-get($theme, 'item-border-color');
+
+            @if $bootstrap-theme {
+                margin: 0 !important;
+                border: none;
+            }
         }
 
         igx-icon {