Remove unused burndown chart CSS

none of the nested styles exist since the move to echarts

ee/app/assets/stylesheets/pages/milestone.scss

@@ -60,76 +60,3 @@
     margin-bottom: 2px;
   }
 }
-
-.burndown-chart {
-  width: 100%;
-  margin: 5px 0;
-
-  .axis {
-    font-size: 12px;
-
-    line,
-    path {
-      fill: none;
-      stroke: $gl-gray-350;
-      shape-rendering: crispEdges;
-    }
-  }
-
-  .axis-label {
-    text {
-      fill: $gl-text-color-secondary;
-    }
-
-    line {
-      stroke: $border-color;
-    }
-  }
-
-  .legend {
-    shape-rendering: crispEdges;
-
-    text {
-      font-size: 13px;
-      fill: $gl-text-color-tertiary;
-    }
-
-    rect {
-      stroke: $border-color;
-      fill: none;
-    }
-  }
-
-  .line {
-    stroke-width: 2px;
-    fill: none;
-
-    &.actual {
-      stroke: $green-500;
-    }
-
-    &.ideal {
-      stroke: $gl-gray-350;
-      stroke-dasharray: 6px 6px;
-    }
-  }
-
-  .focus {
-    circle {
-      fill: $white;
-      stroke: $green-500;
-      stroke-width: 2px;
-    }
-  }
-
-  .chart-tooltip {
-    text {
-      font-size: 12px;
-      fill: $white;
-    }
-
-    rect {
-      fill: $black;
-    }
-  }
-}

ee/app/views/shared/milestones/_burndown.html.haml

@@ -6,7 +6,7 @@
 = warning
 
 - if can_generate_chart?(milestone, burndown)
-  .burndown-chart{ data: { start_date: burndown.start_date.strftime("%Y-%m-%d"),
+  .burndown-chart.mb-2{ data: { start_date: burndown.start_date.strftime("%Y-%m-%d"),
     due_date: burndown.due_date.strftime("%Y-%m-%d"),
     burndown_events_path: expose_url(burndown_endpoint), qa_selector: 'burndown_chart' } }