HTMLMediaElement::removeTextTrack should clear track client
https://bugs.webkit.org/show_bug.cgi?id=114630

Reviewed by Jer Noble.

No new tests, this is clean-up that prevents a potential crash.

* html/HTMLMediaElement.cpp:
(WebCore::HTMLMediaElement::removeTextTrack): Clear the track client.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@148458 268f45cc-cd09-0410-ab3c-d52691b4dbfc