Revert "tcp: Clear sk_send_head after purging the write queue"

This reverts commit e99e7745.

Ben Hutchings writes:

>Sorry, this is the same issue that was already fixed by "tcp: reset
>sk_send_head in tcp_write_queue_purge".  You can drop my version from
>the queue for 4.4 and 4.9 and revert it for 4.14.

Signed-off-by: Sasha Levin <sashal@kernel.org>