[Traits] Remove `ResolvesFutureEntities` trait

[foundrytom]

The core API will be providing a proper mechanism to introspect this aspect of a managers operation:

https://github.com/OpenAssetIO/OpenAssetIO/issues/1209

Closes #67

This poses the question about what do we want to do in the examples:

• Leave it, itll be reet.
• Update to kDerived (or whatever)

[foundrytom]

🤔 mystery option C - how about something like

 # We then check if the manager can tell us where to save the file.
if manager.hasCapability(kResolution):

    working_data = manager.resolve(
           working_ref, TextFileSpecification.kTraitSet,
           ResolveAccess.kWrite, context)

[feltech]

🤔 mystery option C - how about something like

 # We then check if the manager can tell us where to save the file.
if manager.hasCapability(kResolution):

    working_data = manager.resolve(
           working_ref, TextFileSpecification.kTraitSet,
           ResolveAccess.kWrite, context)

With a note that data will be blank if the manager doesn't support it? Though the manager could give an EntityAccessError...