Printer settings: Remove unnecessary separators

• Removes the line in all add printer dialogs.

Screenshots:

Before:
• add printer dialog: http://shortn/_dkhOXdVpWr
• add print server dialog: http://shortn/_Gn4L7Ppngv

After:
• add printer dialog: http://shortn/_aQz95BZyxN
• add print server dialog: http://shortn/_KP8sxGsFZA

BUG=1057030
TEST=manual inspection

Change-Id: Ic6c68253a7d0c01d58d9224afe6147940a9d0e5d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2354966Reviewed-by: Jimmy Gong <jimmyxgong@chromium.org>
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Commit-Queue: Michael Checo <michaelcheco@google.com>
Cr-Commit-Position: refs/heads/master@{#801039}

chrome/browser/resources/settings/chromeos/os_printing_page/cups_add_printer_dialog.html

@@ -10,10 +10,6 @@
 <dom-module id="add-printer-dialog">
   <template>
     <style include="settings-shared">
-      #dialog {
-        /* Force a bottom border regardless of scroll state. */
-        --cr-dialog-body-border-bottom: 1px solid var(--google-grey-300);
-      }
       #dialog [slot=body] {
         padding-inline-end: 0;
         padding-inline-start: 0;