fix get the transition duration parent

[Johann-S]

Fix our build: https://travis-ci.org/twbs/bootstrap/jobs/352733509

/CC @XhmikosR to fix our build and @MartijnCuppens

[MartijnCuppens]

Why would you get the transition duration from the parent if the element has no transition duration?

[Johann-S]

Because as I linked, you cannot get the inherited transition duration on Safari, and in Bootstrap sometimes we use Util.getTransitionDurationFromElement with a child element which do not contain any transition duration

EDIT:
Finally I was wrong we have no use case where we get the transition from the parent so I'll remove a bit of my code