i017 sp:RequiredElements mechanism

Description

The spRequiredElements mechanism allows the policy to state whether a 
header element is required. This seems to provide an alternative to 
specifying required headers in WSDL/Schema. I had originally thoughth that 
this would be useful for other (non-header) elements. After thinking about 
it for a while I now think that WSDL/Schema validation should be used for 
that when needed.

Proposal

Drop the issue.