atm: remove 'struct zatm_t_hist'

The zatm_t_hist structure is not used anywhere in the kernel, but is
exported to user space. As we are trying to eliminate uses of time_t
in the kernel for y2038 compatibility, the current definition triggers
checking tools because it contains 'struct timeval'.

As pointed out by Chas Williams, the only user of this structure was
the ZATM_GETHIST ioctl command that has been removed a long time ago,
and we can remove the structure as well without breaking any user
space.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Chas Williams <3chas3@gmail.com>
Cc: linux-atm-general@lists.sourceforge.net
Signed-off-by: David S. Miller <davem@davemloft.net>