Created by: tlindig
We have variables @zindex-modal and @zindex-modal-background.
This variables were not applied to .modal and .modal-backdrop, but something quite different. This values were used as base for calculation new z-index. But without need.
This patch use @zindex-modal for .modal and @zindex-modal-background for .modal-backdrop, that is what the name suggests.
removed z-index for .modal-dialog, it was not necessary because the parent element (.modal) already has a z-index.