This patch removes several unnecessary fields from the paca:

- next_jiffy_update_tb was simply unused.  Remove trivially.

- The exdsi exception save area was not used.  There were plans to use
  it, but they never seem to have gone anywhere.  If they ever do, we
  can put it back.  Remove from the paca, and from asm-offsets.c

- The default_decr field was used from asm, but was only ever assigned
  the value of tb_ticks_per_jiffy.  Just access tb_ticks_per_jiffy from
  asm directly instead.

Built and booted on POWER5 LPAR and iSeries RS64.
Signed-off-by: David Gibson <dwg@au1.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>