Linux targets: drop fall back to target method for 'make_corefile_notes'

Linux targets: drop fall back to target method for 'make_corefile_notes'

Now that all Linux targets use the regset iterator, the fall back to
the deprecated target method is dropped.

gdb/ChangeLog:

	* linux-nat.c (linux_nat_collect_thread_registers): Remove.
	(linux_nat_make_corefile_notes): Remove.
	(linux_target_install_ops): Do not set target method
	'make_corefile_notes'.
	* linux-tdep.c (struct linux_corefile_thread_data)<collect>:
	Remove field.
	(linux_corefile_thread_callback): Instead of args->collect, call
	linux_collect_thread_registers.
	(linux_make_corefile_notes): Remove 'collect' parameter.  Return
	NULL unless there is a regset iterator.
	(linux_make_corefile_notes_1): Remove.
	(linux_init_abi): Replace reference to linux_make_corefile_notes_1
	by linux_make_corefile_notes.
	* linux-tdep.h (linux_make_corefile_notes): Remove prototype.