gnome-shell: Hide border around user avatars

Hide border that was inherited from the default theme, since the
following upstream commit:
https://gitlab.gnome.org/GNOME/gnome-shell/-/commit/498710c2ec4ea2b3297703a0e9a2ad1168c765aa

Author: jnsh

common/gnome-shell/3.38/sass/_common.scss

@@ -2430,6 +2430,7 @@ StScrollBar {
   background-size: contain;
   color: $osd_fg_color;
   border-radius: 99px;
+  border: 0;
 
   .login-dialog-user-list-item:selected & { color: $selected_fg_color; }