Don't use design metrics with NativeRendering on TextInput

We only set the design metrics property in init(), before the
renderType has been set. Since updateLayout() is called every
time the renderType changes, it should be sufficient to set it
there.

Change-Id: If5b2cb3738e8135195848857a30f4e646bdf1d0b
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>