CRM_Utils_Cache_SqlGroup - Don't encourage infinite recursion

The `deleteGroup(...$clearAll...)` option is heavy-handed and leads to
call-paths that hard to grok.

In the current design of CRM_Core_BAO_Cache (with multi-tier caching), one
does need to clear these things.  But let's not clear everything under the
sun...

CRM/Utils/Cache/SqlGroup.php

@@ -121,12 +121,16 @@ public function getFromFrontCache($key, $default = NULL) {
    * @param string $key
    */
   public function delete($key) {
-    CRM_Core_BAO_Cache::deleteGroup($this->group, $key);
+    CRM_Core_BAO_Cache::deleteGroup($this->group, $key, FALSE);
+    CRM_Core_BAO_Cache::$_cache = NULL; // FIXME: remove multitier cache
+    CRM_Utils_Cache::singleton()->flush(); // FIXME: remove multitier cache
     unset($this->frontCache[$key]);
   }
 
   public function flush() {
-    CRM_Core_BAO_Cache::deleteGroup($this->group);
+    CRM_Core_BAO_Cache::deleteGroup($this->group, NULL, FALSE);
+    CRM_Core_BAO_Cache::$_cache = NULL; // FIXME: remove multitier cache
+    CRM_Utils_Cache::singleton()->flush(); // FIXME: remove multitier cache
     $this->frontCache = array();
   }