* regcache.c (struct regcache_descr): Remove outdated comment.

	* regcache.c (struct regcache_descr): Remove outdated comment.
	(init_regcache_descr): Remove sizeof_raw_register_valid_p
	overallocate hack.
	(regcache_xmalloc): Rename to ...
	(regcache_xmalloc_1): ... this.  Add `readonly_p' parameter.
	Allocate the regcache type accordingly.
	(regcache_xmalloc): New as wrapper around regcache_xmalloc_1.
	(regcache_xfree): Asser the source is also readonly.  Copy sizeof
	cooked registers, not raw.
	(regcache_dup_no_passthrough): Delete.
	(get_thread_arch_regcache): Use regcache_xmalloc_1.
	* h8300-tdep.c (h8300_push_dummy_call): Tweak comment to not
	mention obsolete write_register_bytes.
	* regcache.h (regcache_dup_no_passthrough): Delete declaration.