Should we drop __SYCL_INLINE_VER_NAMESPACE(_V1)?

[bader]

This macro was originally introduced by the name __SYCL_INLINE/__SYCL_INLINE_NAMESPACE to support both standards SYCL 1.2.1 and SYCL 2020 APIs. Later it was repurposed to handle ABI changes, but I don't see any uses of this mechanism in the project. ABI changes are handled by the policy exposed here: https://github.com/intel/llvm/blob/sycl/sycl/doc/developer/ABIPolicyGuide.md.

[bader]

__SYCL_INLINE_VER_NAMESPACE has been removed by #10510.