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.
Merge pull request #12954 from owncloud/hhvm-apc-clear

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