UBIFS: Remove unused variables in ubifs_budget_space

I found two variables in ubifs_budget_space declared but not
use. This state remains since the first commit 1e5176. So just
remove them.

Signed-off-by: hujianyang <hujianyang@huawei.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>