TextField: Remove placeholder text fade out transition

It's disturbing for the user that is focusing in the text she's typing.
The transition feels like the placeholder text is going to stay for a
fraction of a second, which is an unnecessary feeling.

Change-Id: I44589486dcb3ca363221ed4dabb1752e9e3b6550
Reviewed-by: Caroline Chao <caroline.chao@digia.com>