HHVM: In APC cache clear, only request the cache key in APCIterator.

The default value of the $format parameter of the APCIterator constructur is
APC_ITER_ALL which instructs the iterator to provide all available information
on cache values being iterated over. Only the key value is necessary for
matching and deletion via apc_delete(), though.

This prevents a "Format values FILENAME, DEVICE, INODE, MD5, NUM_HITS, MTIME,
CTIME, DTIME, ATIME, REFCOUNT not supported yet." notice on HHVM.

@icewind1991 @DeepDiver1975 @LukasReschke 

Tested apc.php against HHVM and apcu.php against PHP.

Also see https://github.com/facebook/hhvm/blob/master/hphp/system/php/apc/APCIterator.php