The BindingPointer will call onDestroy when all references are removed, no
need to do it explicitly.

BUG=angleproject:3070

Change-Id: Ic5dd4da8f73e204d26d9d3a3c6e1eff9fded5411
Reviewed-on: https://chromium-review.googlesource.com/c/1409868Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>