Remove linux_proc_pid_get_ns

Remove linux_proc_pid_get_ns

This commit removes linux_proc_pid_get_ns, and updates its only
caller to use linux_ns_same instead.

gdb/ChangeLog:

	* linux-thread-db.c (nat/linux-namespaces.h): New include.
	(check_pid_namespace_match): Use linux_ns_same rather than
	linux_proc_pid_get_ns to spot PID namespace mismatches.
	* nat/linux-procfs.h (linux_proc_pid_get_ns): Remove.
	* nat/linux-procfs.c (linux_proc_pid_get_ns): Likewise.