Cleanup modal z-index

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.