net: u64_stats_sync: Remove functions without user

Commit 67db3e4b ("tcp: no longer hold ehash lock while calling
tcp_get_info()") removes the only users of u64_stats_update_end/begin_raw()
without removing the function in header file.

Remove no longer used functions.

Cc: Eric Dumazet <edumazet@google.com>
Signed-off-by: Anna-Maria Gleixner <anna-maria@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>