[data-shapes] remove `owl:imports sh:` from examples (#240)

VladimirAlexiev has just created a new issue for https://github.com/w3c/data-shapes:

== remove `owl:imports sh:` from examples ==
https://w3c.github.io/data-shapes/shacl/#sparql-prefixes gives an example with `owl:imports sh: ;`
Others copy this example, eg in the electrical CIM/CGMES.
- as explained in https://github.com/Sveino/Inst4CIM-KG/issues/157, I see no reason to import `sh:` in SHACL files.
- whereas it's hurtful in SHACL-SHACL validation, as discussed in https://github.com/ISAITB/shacl-validator/issues/15


Please view or discuss this issue at https://github.com/w3c/data-shapes/issues/240 using your GitHub account


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