Clean up `layouts/partials/breadcrumb.html`

[chalin]

• What is the spb-1 class applied to the <ol> element in the following excerpt:

  <nav aria-label="breadcrumb" class="d-none d-md-block d-print-none">
      <ol class="breadcrumb spb-1">
    	  {{ template "breadcrumbnav" (dict "p1" . "p2" .) }}
      </ol>
  </nav	>

  @LisaFC @emckean @bep any idea?
  It isn't in the generated styles or in the JS (as an anchor) AFAICT, so I'd vote to drop it.
  FYI, this file hasn't changed since the original commit 4 years ago: https://github.com/google/docsy/blame/master/layouts/partials/breadcrumb.html
• Drop unnecessary whitespace from the partial and untabify. See the excerpt above. - Show breadcrumbs on mobile too #784
  Note the extra inside the nav closing tag.
• Ensure it doesn't generate any unnecessary whitespace: e.g., use {{ ... -}} - Show breadcrumbs on mobile too #784