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.