Don't display taxonomy term reference field headers when field label display is set to "hidden"

Problem/Motivation

In #1106344: Taxonomy term reference field headers always should be rendered with a HTML header (invisible or not), taxonomy term reference field headers whose field label display was set to "- Hidden -" were changed so they would be visible to screenreaders.

But in #2227601: [Follow-up] Add a "Visually-hidden" field label formatter, a new "- Visually Hidden -" label display was added to let site-builders explicitly choose to hide labels but make them visible to screenreaders.

The purpose of this issue is to revert the changes made in #1106344: Taxonomy term reference field headers always should be rendered with a HTML header (invisible or not). Input from @mgifford would be greatly appreciated, as he was the primary driver behind #1106344. Here's a link to the commit: http://cgit.drupalcode.org/drupal/commit/?id=e04d6380cdfde61fbdf6dc3eec7...

Proposed resolution

Make Bartik's taxonomy term reference field label consistent with other field formatter labels and other themes by removing it from the markup when the field label display is set to "- Hidden -".

Remaining tasks

1. Get sign-off from @mgifford
2. Write a patch
3. Review and RTBC
4. Commit

User interface changes

Bartik's taxonomy term reference field labels are removed from the markup when the label display is set to "- Hidden -", similar to other fields and the rest of core.

API changes

None.