use bootstrap tooltips instead of own solution (#741)

assets/sass/app.scss

@@ -38,5 +38,4 @@ footer.main-footer {
 @import 'dashboard';
 @import 'navbar';
 @import 'daterangepicker';
-@import 'tooltip';
 @import 'skins';

assets/sass/tooltip.scss

@@ -1,36 +0,0 @@
-/*
- * This file is part of the Kimai time-tracking app.
- *
- * For the full copyright and license information, please view the LICENSE
- * file that was distributed with this source code.
- */
-
-/*
- * These styles are not yet used and need some love before actively promoted in the UI.
- * They could be used in: templates/macros/widgets.html.twig::label
- */
-
-[data-tooltip] {
-    position: relative;
-    overflow: visible;
-}
-[data-tooltip]:after {
-    transition: all .2s ease;
-    content: attr(data-tooltip);
-    position: absolute;
-    top: -24px;
-    left: 50%;
-    transform: translateX(-50%);
-    background: $gray;
-    color: #fff;
-    padding: 2px 12px;
-    pointer-events: none;
-    white-space: nowrap;
-    font-size: 11px;
-    line-height: 18px;
-    border-radius: $border-radius-small;
-    z-index: 2;
-}
-[data-tooltip]:not(:hover):after {
-    opacity: 0;
-}