Leverage CSS vs. JS for pipeline padding

- Match class use in graph_component.vue to surrounding approach
- Remove now unused graph width mixin

app/assets/javascripts/pipelines/mixins/graph_width_mixin.js

-import { debounceByAnimationFrame } from '~/lib/utils/common_utils';
-import { LAYOUT_CHANGE_DELAY } from '~/pipelines/constants';
-
-export default {
-  debouncedResize: null,
-  sidebarMutationObserver: null,
-  data() {
-    return {
-      graphLeftPadding: 0,
-      graphRightPadding: 0,
-    };
-  },
-  beforeDestroy() {
-    window.removeEventListener('resize', this.$options.debouncedResize);
-
-    if (this.$options.sidebarMutationObserver) {
-      this.$options.sidebarMutationObserver.disconnect();
-    }
-  },
-  created() {
-    this.$options.debouncedResize = debounceByAnimationFrame(this.setGraphPadding);
-    window.addEventListener('resize', this.$options.debouncedResize);
-  },
-  mounted() {
-    this.setGraphPadding();
-
-    this.$options.sidebarMutationObserver = new MutationObserver(this.handleLayoutChange);
-    this.$options.sidebarMutationObserver.observe(document.querySelector('.layout-page'), {
-      attributes: true,
-      childList: false,
-      subtree: false,
-    });
-  },
-  methods: {
-    setGraphPadding() {
-      // only add padding to main graph (not inline upstream/downstream graphs)
-      if (this.type && this.type !== 'main') return;
-
-      const container = document.querySelector('.js-pipeline-container');
-      if (!container) return;
-
-      this.graphLeftPadding = container.offsetLeft;
-      this.graphRightPadding = window.innerWidth - container.offsetLeft - container.offsetWidth;
-    },
-    handleLayoutChange() {
-      // wait until animations finish, then recalculate padding
-      window.setTimeout(this.setGraphPadding, LAYOUT_CHANGE_DELAY);
-    },
-  },
-};