* regcache.h: Update comments describing the regcache_cpy family

* regcache.h: Update comments describing the regcache_cpy family
of functions.
(regcache_save, regcache_restore): Delete declaration.
(regcache_save_no_passthrough): Delete declaration.
(regcache_restore_no_passthrough): Delete declaration.
* regcache.c (regcache_save): Delete function.
(regcache_save_no_passthrough): Delete function.
(regcache_restore): Delete function.
(regcache_restore_no_passthrough): Delete function.