Remove deprecated core file functions

There are no more callers to deprecated_add_core_fns, now that I have
removed the usage from CRIS and ARM/NetBSD.  So this patch cleans up
all the related code and makes corelow.c a lot more readable.

gdb/ChangeLog:

2020-03-12  Christian Biesinger  <cbiesinger@google.com>

	* corelow.c (sniff_core_bfd): Remove.
	(class core_target) <m_core_vec>: Remove.
	(core_target::core_target): Update.
	(core_file_fns): Remove.
	(deprecated_add_core_fns): Remove.
	(default_core_sniffer): Remove.
	(sniff_core_bfd): Remove.
	(default_check_format): Remove.
	(gdb_check_format): Remove.
	(core_target_open): Update.
	(core_target::get_core_register_section): Update.
	(get_core_registers_cb): Update.
	(core_target::fetch_registers): Update.
	* gdbcore.h (struct core_fns): Remove.
	(deprecated_add_core_fns): Remove.
	(default_core_sniffer): Remove.
	(default_check_format): Remove.