SoftInputWindow: Remove clamping logic

With current IMEs this has no effect because the DecorView always extends to the full bounds (unlike before Lollipop, where the IME did not extend under the navigation bar by default).

The only effect this had in practice was to throw away the raw coordinates and history, when dispatching touch events from the very bottom of the screen.

Fixes: 265611882
Test: n/a (deletes special casing)
Change-Id: I914c2347f60246d40e0db3a77cbae13b77d88958