atomics: Revert addition of comment header to spin_unlock_wait()

There is still considerable confusion as to the semantics of
spin_unlock_wait(), but there seems to be universal agreement that
it is not that of a lock/unlock pair.  This commit therefore removes
the comment added by 6016ffc3 ("atomics: Add header comment so
spin_unlock_wait()") in order to prevent at least that flavor of
confusion.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>