staging: zcache: delete it

zcache is obsolete and not used anymore, Bob Liu has rewritten it and is
submitting it for inclusion through the main -mm tree, as it should have
been done in the first place...

Cc: Bob Liu <lliubbo@gmail.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Kyungmin Park <kmpark@infradead.org>
Cc: Wanpeng Li <liwanp@linux.vnet.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>