Remove deprecated oneapi extensions (partially assigned, additional assigneess welcome)

[KornevNikita]

Describe the bug

Currently we have 3 deprecated extensions that have been deprecated in favor of core spec / khr extensions.
We need to:

1. deprecate the API (although I assume that it's deprecated already, but need to double-check and deprecate the leftover if any).
2. guard all deprecated stuff with #ifndef __INTEL_PREVIEW_BREAKING_CHANGES to remove in the next ABI-break window.
3. All 3 extensions specifications are duplicated - there is a file in extensions/supported & extensions/deprecated. Usually we move specifications to the removed folder, but we can't do this until the API is removed. So I suggest to add a comment like this to the file in the supported dir:

// __INTEL_PREVIEW_BREAKING_CHANGES: remove this file along with the feature during the next ABI-break window.

and a comment like this to the file in the deprecated dir:

// __INTEL_PREVIEW_BREAKING_CHANGES: during the ABI-break window update the status of this specification according to https://github.com/intel/llvm/blob/sycl/sycl/doc/extensions/template.asciidoc and move it to the `removed` folder along with the removal of this feature.

Note - I suggest to remove each spec in a separate PR to avoid huge patches.

To reproduce

1. Include a code snippet that is as short as possible
2. Specify the command which should be used to compile the program
3. Specify the command which should be used to launch the program
4. Indicate what is wrong and what was expected

Environment

• OS: [e.g Windows/Linux]
• Target device and vendor: [e.g. Intel GPU]
• DPC++ version: [e.g. commit hash or output of clang++ --version]
• Dependencies version: [e.g. the output of sycl-ls --verbose]

Additional context

No response

[softwaresat]

Hi! I'm an ECE student at UT Austin and in Codepath's AI301 Open Source Capstone program and I would love to work on this issue. However, I have a 2–3 week development window. Since each deprecated issue is intended for a separate PR, which of the three would be the smallest and most suitable for a first-time contributor? I’d like to take ownership of that individual part first than the entire issue.

[KornevNikita]

@softwaresat hi sorry for the delay, I was on vacation. I'd say that sycl_ext_oneapi_default_context.asciidoc should be the easiest to start with.

[softwaresat]

Thanks! I’ll work on removing sycl_ext_oneapi_default_context as a separate PR. I’ll start by identifying the deprecated API and feature-test macro, guarding them for preview breaking changes, moving the specification to the removed directory, and adding the relevant tests. Please assign me to this part if possible.

[softwaresat]

@KornevNikita Hi, thank you for pointing me toward this extension and giving me the opportunity to work on it. I’ve completed the changes and opened a PR. Please let me know if there are any changes or additional tests you’d like me to add.