s390/time: remove get_tod_clock_ext()

Remove get_tod_clock_ext() and the STORE_CLOCK_EXT_SIZE define. This
enforces all users of the existing low level functions to use a union
tod_clock.

This way there is now a compile time check for the correct time and
therefore also if the size of the argument matches what will be
written to by the STORE CLOCK EXTENDED instruction.

Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>