net: remove unused sock_enable_timestamps

This function was introduced in commit 783da70e ("net: add
sock_enable_timestamps"), with one caller in rxrpc.

That only caller was removed in commit 7903d443 ("rxrpc: Don't use
received skbuff timestamps").

Signed-off-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Jason Xing <kerneljasonxing@gmail.com>
Link: https://patch.msgid.link/20250609153254.3504909-1-willemdebruijn.kernel@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>