PHP TextInputWidget: Remove remaining type 'search' specific code
PHP TextInputWidget: Remove remaining type 'search' specific code

TextInputWidget `type='search'` support was removed in JS in
I4fbe336fafaaa6c9e228802e4e9463d11009be72 but some PHP code remained.
Let's get rid of it.

Change-Id: I8ae76d24582493dc069578137d53c497016f0a85

wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui