Remove `sip::Hasher::short_write`.

Remove `sip::Hasher::short_write`.

`sip::Hasher::short_write` is currently unused. It is called by
`sip::Hasher::write_{u8,usize}`, but those methods are also unused,
because `DefaultHasher`, `SipHasher` and `SipHasher13` don't implement
any of the `write_xyz` methods, so all their write operations end up
calling `sip::Hasher::write`.

(I confirmed this by inserting a `panic!` in `sip::Hasher::short_write`
and running the tests -- they all passed.)

The alternative would be to add all the missing `write_xyz` methods.
This does give some significant speed-ups, but it hurts compile times a
little in some cases. See #69152 for details. This commit does the
conservative thing and doesn't change existing behaviour.