popover-class-indicator (issue #14607)

Addresses issue #14607 by adding 'no-title' to the popover tip.

If you'd prefer a different class name instead of `no-title` please let me know. The `.find('.popover-title').hide()` portion of this line could actually be removed, in favor of hiding the title via LESS and the new `no-title` class... if you think that change would be worthwhile, I could throw that in there.

Additionally, I tested this minor change via the `bootstrap/js/tests/visual/popover.html` and by running `grunt test`, however I did not commit the dist files that were generated. Let me know if you'd like those to be included and I will do so (along with a squash to make it one commit). Thanks!