Remove cpu-plugin.c

After commit 999d6dff cpu-plugin.c is no longer begin used.

	* Makefile.am (ALL_MACHINES): Remove cpu-plugin.lo.
	(ALL_MACHINES_CFILES): Remove cpu-plugin.c.
	* Makefile.in: Regenerate.
	* cpu-plugin.c: Remove.
	* archures.c (enum bfd_architecture): Remove bfd_arch_plugin.
	(bfd_plugin_arch): Remove declaration.
	* bfd-in2.h: Regenerate.
	* po/SRC-POTFILES.in: Regenerate.