Drop 'regset_from_core_section' gdbarch method

Drop 'regset_from_core_section' gdbarch method

Now that all instances of the regset_from_core_section gdbarch method
have been replaced by the new iterator method, delete the obsolete
method from the gdbarch interface.  Adjust all invocations and
references to it.

gdb/ChangeLog:

	* gdbarch.sh (regset_from_core_section): Remove gdbarch method.
	* gdbarch.c: Regenerate.
	* gdbarch.h: Likewise.
	* corelow.c (sniff_core_bfd): Drop presence check for deleted
	gdbarch method 'regset_from_core_section'.
	(get_core_register_section): Remove handling for the case that
	regset == NULL and regset_from_core_section is defined.
	(get_core_registers): Drop check for deleted method.
	* procfs.c (procfs_do_thread_registers): Adjust comment.