Summary:
The patch cleans up a few things in `CompactionJob::SubcompactionState`:

* Instead of using both the member initializer list and in-class initializers (and
sometimes both at the same time for the same member), the struct now uniformly
uses the latter to initialize integer members.
* The default parameter value for the constructor parameter `size` is removed.
* The explicitly deleted copy operations are removed, since they are implicitly deleted
anyways because of the `unique_ptr` members.
* The handwritten move operations, which did not move the member `c_iter` and
were not declared `nothrow`, are removed. Note that with the user-declared copy
operations gone (see the previous item), we can rely on the compiler to (correctly)
generate these methods.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7322

Test Plan: `make check`

Reviewed By: siying

Differential Revision: D23382408

Pulled By: ltamasi

fbshipit-source-id: a4ae5af150161c50ff7bdc07fa145482d0150bfe