gpu: sycl: eltwise: Reduce size of kernel arguments

[sgeor255]

Description

This MR slightly refactors the eltwise SYCL kernel to remove some unused data that was stored in the conf struct passed to the kernel. As a result, the size of the struct is reduced, allowing the kernel to run on some specific devices with a lower kernel argument size limit.

Checklist

General

• [X] Do all unit and benchdnn tests (make test and make test_benchdnn_*) pass locally for each commit?
• [X] Have you formatted the code using clang-format?