[Spec change] Remove reference to OpenCL 1.2 memory model

[sycl-issue-bot]

Please review whether KhronosGroup/SYCL-Docs#714 by @Pennycook requires any changes to the CTS.

If changes are required: Open a new PR addressing the changes and link it to this issue.
If no changes are required: Close this issue and proceed with the spec PR.

[Pennycook]

I'd argue this is editorial, but didn't want to use the tag in case somebody disagreed. Either way, this is unobservable so doesn't require CTS changes.