xen/sched: Remove {init,destroy}_domain() interfaces

xen/sched: Remove {init,destroy}_domain() interfaces

These hooks have one single caller (sched_{init,destroy}_domain()
respectively) and are all identical (when implemented).

Previous changes have ensured that only real domains reach these functions, so
ASSERT() that system domains are not seen. Call sched_{alloc,free}_domdata()
directly, and handle d->sched_priv directly.

The net diffstat is:
  add/remove: 0/8 grow/shrink: 1/7 up/down: 7/-335 (-328)
  function                                     old     new   delta
  sched_destroy_domain                         130     137      +7
  sched_init_domain                            138     137      -1
  rt_dom_destroy                                 6       -      -6
  null_dom_destroy                               6       -      -6
  csched_dom_destroy                             9       -      -9
  csched2_dom_destroy                            9       -      -9
  sched_rtds_def                               264     248     -16
  sched_null_def                               264     248     -16
  sched_credit_def                             264     248     -16
  sched_credit2_def                            264     248     -16
  sched_arinc653_def                           264     248     -16
  ops                                          264     248     -16
  rt_dom_init                                   52       -     -52
  null_dom_init                                 52       -     -52
  csched_dom_init                               52       -     -52
  csched2_dom_init                              52       -     -52

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>
Acked-by: Meng Xu <mengxu@cis.upenn.edu>
Reviewed-by: Dario Faggioli <dfaggioli@suse.com>