style: Remove `max-height` from the `input` style.

Some pages, like Google, want to set height explicitly.
auto merge of #3602 : pcwalton/servo/max-height-input, r=jdm

Some pages, like Google, want to set height explicitly.

r? @jdm
layout: `white_space::pre` should not yield ignorable whitespace.

Improves the Google home page.