prefer_only example is out of date

[dkolsen-pgi]

In P0443R13, section 2.4.2 "Struct prefer_only", the code in the example is out of date. I think the example is sound, it just uses the old name of something that has been renamed, and uses a couple properties that have been removed from the paper.

The necessary changes are:

• Change executor to any_executor.
• Delete references to obsolete properties execution::single and execution::oneway.
• With those two properties gone, some calls to std::require now have no properties to require. Those calls to std::require should be removed.

[jaredhoberock]

Fixed in resolution to #486