tcp: Remove the unused api

Last tcp_write_queue_head() use was removed in commit
114f39fe

 ("tcp: restore autocorking"), so remove it.

Signed-off-by: Tao Chen <chentao3@hotmail.com>
Link: https://lore.kernel.org/r/SYZP282MB33317DEE1253B37C0F57231E86029@SYZP282MB3331.AUSP282.PROD.OUTLOOK.COM
Signed-off-by: Jakub Kicinski <kuba@kernel.org>