[NFC] Drop code related to error_category_for

[AlexeySachkov]

That API was removed from the SYCL 2020 specification in KhronosGroup/SYCL-Docs#431

[keryell]

Thanks.
I am unsure this is really [NFC] as you are removing some functionality.

[AlexeySachkov]

Thanks. I am unsure this is really [NFC] as you are removing some functionality.

As you can see, I've only changed a single file which defined those "helpers" - that means that they have never been in use and that was essentially a dead code. In that case this probably satisfies [NFC] tag.

Note: I'm fine with the tag removal. I can do that myself, or it can be done by a maintainer if squash strategy is used when accepting the PR. Second note: without squash on accept, removal of the tag from the existing commit will require rebase and force-push from my side

[bader]

We use merge strategy in this repository.

Thanks. I am unsure this is really [NFC] as you are removing some functionality.

As you can see, I've only changed a single file which defined those "helpers" - that means that they have never been in use and that was essentially a dead code. In that case this probably satisfies [NFC] tag.

I'm fine merging with existing PR title. @keryell?