GC gdb/thread.c:current_thread_cleanup_chain

GC gdb/thread.c:current_thread_cleanup_chain

Commit 803bdfe43083475c7df3db38dc96f4e20d05457d ("Don't delete
thread_info if refcount isn't zero") eliminated
restore_current_thread_ptid_changed, so current_thread_cleanup_chain
is no longer necessary either.

gdb/ChangeLog:
2017-04-10  Pedro Alves  <palves@redhat.com>

	* thread.c (struct current_thread_cleanup) <next>: Delete field.
	(current_thread_cleanup_chain): Delete.
	(restore_current_thread_cleanup_dtor)
	(make_cleanup_restore_current_thread): Remove references to
	current_thread_cleanup_chain.