[fir] Remove `fir.unbox` operation

`fir.unbox` operation is an old operation that is no longer required.
There are couple of other operations that can be used to extract
information from a `fir.box` such as `fir.box_rank`, `fir.box_addr`,
`fir.box_dims`.

This was found during the upstreaming process.

Reviewed By: awarzynski

Differential Revision: https://reviews.llvm.org/D113581