pid: kill the obsolete PIDTYPE_PID code in transfer_pid()

transfer_pid() must be never called with pid == PIDTYPE_PID,
new_leader->thread_pid should be changed by exchange_tids().

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Link: https://lore.kernel.org/r/20240202131255.GA26025@redhat.com


Signed-off-by: Christian Brauner <brauner@kernel.org>