copy_signal() cleanup: clean thread_group_cputime_init()

Remove unneeded initializations in thread_group_cputime_init() and in
posix_cpu_timers_init_group().  They are useless after kmem_cache_zalloc()
was used in copy_signal().

Signed-off-by: Veaceslav Falico <vfalico@redhat.com>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Cc: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>