mark "cache-file" as ancient and remove all code implementing it

mark "cache-file" as ancient and remove all code implementing it

"cache-file" was already documented as intended for testing
purposes only and not to be used, so we can remove it without
waiting.  this commit marks the option as "ancient", and
removes all the documentation and implementing code, including
dns_cache_setfilename() and dns_cache_dump().

it also removes the documentation for the '-x cachefile`
parameter to named, which had already been removed, but the man
page was not updated at the time.