garbage collect target_decr_pc_after_break

garbage collect target_decr_pc_after_break

record-btrace was the only target making use of this, and it no longer
uses it.

gdb/ChangeLog:
2015-03-04  Pedro Alves  <palves@redhat.com>

	* target.h (struct target_ops) <to_decr_pc_after_break>: Delete.
	(target_decr_pc_after_break): Delete declaration.
	* target.c (default_target_decr_pc_after_break)
	(target_decr_pc_after_break): Delete.
	* linux-nat.c (check_stopped_by_breakpoint, linux_nat_wait_1): Use
	gdbarch_decr_pc_after_break instead of target_decr_pc_after_break.
	* linux-thread-db.c (check_event): Likewise.
	* infrun.c (adjust_pc_after_break): Likewise.
	* darwin-nat.c (cancel_breakpoint): Likewise.
	* aix-thread.c (aix_thread_wait): Likewise.
	* target-delegates.c: Regenerate.