Fix singleton mode with bootstrap5:

Seems that negative margin is no more needed with the recent changes.

src/plugins/singleton/singleton.scss

@@ -18,13 +18,6 @@
             .chatbox {
                 margin: 0;
                 position: relative;
-                margin-left: -15px;
-                @media screen and (max-width: $mobile-portrait-length) {
-                    margin-left: 0;
-                }
-                @include media-breakpoint-down(sm) {
-                    margin-left: 0;
-                }
             }
         }
     }