[FIX] web: restore saas modals

This commit basically reverts 4974f8edb7
The fix was unstable (as restoring a feature that was not there
before release, even if originally merged before). This was in
particular breaking saas modals as lots of them are still opened
using 8.0 system... and opening them this way might make them appear
below their own backdrop in the new system the mentioned commit
restored.

We will think about that feature again for a future version.

addons/web/static/src/less/bootstrap_overridden.less

@@ -83,7 +83,10 @@
 //
 // Note that the variable @zindex-modal-background may be renamed/removed in
 // newer versions of bootstrap.
-@zindex-modal-background: @zindex-modal;
+//
+// TODO the following code was disabled because it is saas-incompatible
+//
+// @zindex-modal-background: @zindex-modal;
 
 //== Navs
 @nav-link-padding: 10px 15px;

addons/web/static/src/less/modal.less

@@ -79,6 +79,9 @@
 
 // Temporary fix for modals which are not instantiated thanks to the Dialog
 // JS classes (deprecated case) (see bootstrap_overridden.less) + Frontend.
-:not(body) > .modal {
-    z-index: @zindex-modal-background + 1;
-}
+//
+// TODO the following code was disabled because it is saas-incompatible
+//
+// :not(body) > .modal {
+//     z-index: @zindex-modal-background + 1;
+// }