[FIX] web: avoid horizontal scroll on kanban mobile

Steps to reproduce:
1. Open 'Project' on mobile
2. You can scroll x while the content fit the screen.

This issue occured because of negative margins on
'o_kanban_manage_toggle_button' class.
In general, using negative margins should be avoided.

So in this commit we simply avoid to have negative margins by
removing unwanted borders. It keeps the same design.

Similar issue:
https://github.com/odoo/enterprise/commit/805ab4d4a329c0fdf7d1cade6c5d7e8428f3a634

closes odoo/odoo#42518

X-original-commit: 38702215e6dff4e562a389ec03cdeb8683cd111c
Signed-off-by: Adrien Dieudonné (adr) <adr@odoo.com>

addons/web/static/src/scss/kanban_dashboard.scss

@@ -34,7 +34,7 @@
 
             // Arbitrary value to place the dropdown-menu exactly below the
             // dropdown-toggle (height is forced so that it works on Firefox)
-            top: $o-kanban-manage-toggle-height - 1;
+            top: $o-kanban-manage-toggle-height;
 
             > div:not(.o_no_padding_kanban_colorpicker) {
                 padding: 3px 0 3px 20px;

addons/web/static/src/scss/utils.scss

@@ -278,10 +278,9 @@
     margin-top: 0;
 }
 @mixin o-kanban-dropdown($padding-base: $o-kanban-inside-vgutter) {
-    margin: -1px -1px auto auto; // compensate border
     padding: $padding-base/2 $padding-base;
-    border: 1px solid transparent;
-    border-bottom: none;
+    border: none;
+    border-left: 1px solid transparent;
     vertical-align: top;
     color: $body-color;