Fix leaks when pruning inferiors.

Fix leaks when pruning inferiors.

Valgrind detects various inferior related leaks, such as:
  ==31877== 5,530 (56 direct, 5,474 indirect) bytes in 1 blocks are definitely lost in loss record 7,131 of 7,355
  ==31877==    at 0x4C2E18C: calloc (vg_replace_malloc.c:760)
  ==31877==    by 0x23E580: xcalloc (alloc.c:100)
  ==31877==    by 0x4794A9: xcnewvec<void*> (poison.h:158)
  ==31877==    by 0x4794A9: registry_alloc_data(registry_data_registry*, registry_fields*) (registry.c:51)
  ==31877==    by 0x3A537C: inferior_alloc_data (inferior.c:43)
  ==31877==    by 0x3A537C: inferior::inferior(int) (inferior.c:92)
  ==31877==    by 0x3A5426: add_inferior_silent(int) (inferior.c:98)
  ==31877==    by 0x3A5530: add_inferior(int) (inferior.c:122)
  ...

Origin of the leaks is in prune_inferiors: prune_inferiors is first removing
the inferior to prune from the inferior list, then calls delete_inferior.
But delete_inferior will only really destroy the inferior when it finds
it into the inferior list.
As delete_inferior is removing the inferior to delete from the inferior list,
ensure prune_inferiors only calls delete_inferior, without touching the
inferior list.

gdb/ChangeLog
2019-12-05  Philippe Waroquiers  <philippe.waroquiers@skynet.be>
	* inferior.c (prune_inferiors):  Only call delete_inferior.
	Do not modify the inferior list.