mod_cache: Remove unimplemented CacheForceCompletion directive.

* modules/cache/mod_cache.h: Remove DEFAULT_CACHE_COMPLETION #define and
  complete-related vars.
* modules/cache/mod_cache.c: Ditto.
* modules/cache/mod_disk_cache.c: Clarify error message; remove unused
  complete var (unrelated to CacheForceCompletion removal).
* docs/manual/mod/mod_cache.xml.ja, docs/manual/mod/mod_cache.xml.ko,
  docs/manual/mod/mod_cache.xml: Remove references to CacheForceCompletion.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@125573 13f79535-47bb-0310-9956-ffa450edef68