[SYCL][NFC] Optimize vector_byte.cpp E2E test

[AlexeySachkov]

Removed SYCL_SIMPLE_SWIZZLES macro definition, because those swizzles are not used by the test.

Defining SYCL_SIMPLE_SWIZZLES has significant impact on compilation time: the test compiles about twice faster without it on my local machine.

Also fixed warnings about unused variable.

[maarquitos14]

LGTM. Just a couple of non-blocking questions:

• Was the macro used before and at some point it stopped being necessary? Or it was never necessary?
• Is this something that might be happening in other tests? 2x speedup is a very significant speedup, so if this is indeed happening in other tests, we should replicate the solution there.

[AlexeySachkov]

• Was the macro used before and at some point it stopped being necessary? Or it was never necessary?

I think that in this particular test it was introduced with the initial commit, but haven't been really used.

• Is this something that might be happening in other tests? 2x speedup is a very significant speedup, so if this is indeed happening in other tests, we should replicate the solution there.

It could, but grep shows that there are only two other E2E tests which use the macro: swizzle_op and vector_operators. I think that both of them can be refactored to still use swizzles (methods like .lo()/.hi(), .odd()/.even() are always available) but avoid defining the macro.
I also did some cleanup related to the macro before in #10137 (and maybe in other PRs which I don't remember).