include/asm-generic: Remove spin_is_locked() comment

The WARN_ON_SMP() comment header suggests using spin_is_locked() to
check for locks being held.  But these days we prefer lockdep_assert_held(),
so this commit removes that suggestion.

Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: <linux-arch@vger.kernel.org>