refactor: remove mixins.less. flex browser-prefixes not needed

[lokesh]

As I've been looking at the current the CSS organization I've been finding some low hanging fruit for clean up. In following process, in the future I'll create a Github Issue that captures this work.

This PR deletes static/css/less/mixins.less

Technical

In the file were two mixins:

.display-flex() {
  display: -webkit-box;
  display: -ms-flexbox;
  display: -webkit-flex;
  display: flex;
}
.shell-btn--active() {
  background-color: @white;
  border: 2px solid @primary-blue;
  color: @primary-blue;
}

• .display-flex() - Safari, the last browser to do so, dropped the browser prefix requirement for flex in 2014.
• .shell-btn--active() - This was only used in one location. I moved the code inline.

Stakeholders

@jimchamp @cdrini @mekarpeles