ceph-kvstore-tool: define a noexcept non-default ctor for Deleter

ceph-kvstore-tool: define a noexcept non-default ctor for Deleter

the deleter of a unique_ptr<> should be value-initialized if we use
`unique_ptr()` for constructing the unique_ptr, but somehow, `Deleter`
does have a user-defined constructor which prevents the compiler from
creating a default constructor which could have made Deleter default
constructible.

in this change, a constructor accepts no arguments is explictly defined
to satisfy the requirements for creating `db` using `unique_ptr<>()`.

this change is not cherry-picked from master, as we don't define a
constructor at all for Deleter, so it is *always* value-initialized.

Signed-off-by: Kefu Chai <kchai@redhat.com>