gdb: Remove cleanup from linux_nat_target::follow_fork

gdb: Remove cleanup from linux_nat_target::follow_fork

Remove cleanup from linux_nat_target::follow_fork, instead add a new
unique_ptr specialisation for holding lwp_info pointers and use this
to ensure the pointer is cleaned up when needed.

gdb/ChangeLog:

	* linux-nat.c (delete_lwp_cleanup): Delete.
	(struct lwp_deleter): New struct.
	(lwp_info_up): New typedef.
	(linux_nat_target::follow_fork): Delete cleanup, and make use of
	lwp_info_up.