[IMP] web_editor: remove dead '_onInput' code

The 'insertLineBreak' case in `onInput` is dead code because any event
that might cause it (pressing Shift+Enter) first triggers `onKeydown`,
which calls `ev.preventDefault()` and thus prevents it from reaching
`onInput`.

Task-3301232

closes odoo/odoo#120854

Signed-off-by: David Monjoie (dmo) <dmo@odoo.com>