InputDevice: remove hasButtonUnderPad

This API was added back in 2013 [0], but with the new touchpad stack it
doesn't return the correct value. It's also marked @hide and unused, so
instead of doing the plumbing let's just get rid of it.

[0]: change I2cc10457595a861b707c0394bd435d60efa8405b

Bug: 419216111
Test: $ atest --host inputflinger_tests
Test: $ m checkinput
Flag: EXEMPT refactor
Change-Id: Ifbea16642d8348759d9681a60df5096bb18d7e40