Revert "revalidator: Use xcache when revalidation is required."

This reverts commit a48c85b2d672505b89e488d28066538705b94942. The commit
was causing intermittent testsuite failures and unexpected re-install of
stale mac-learning entries.

VMware-BZ: 1268574

Reported-by: Len Gao <leng@vmware.com>
Signed-off-by: Joe Stringer <joestringer@nicira.com>
Acked-by: Alex Wang <alexw@nicira.com>