Remove redundant when_null_skip= kwarg

[pivotaljohn]

In the design of ytt Validations, we proposed that nullable Data Values would require the configuration to skip validation should the value remain null.
This has been implemented as the when_null_skip= keyword argument.

However, through adjustments in design made during implementation and usability testing (#707), we've determined that this mechanism is not needed in practice:

• when a Data Value has been marked as @schema/nullable the when_null_skip= is defaulted to True;
• in practice, when a value is null, the only interesting validation rule is to assert that it not be null (e.g. not_null=True);
• so, the only time when when_null_skip= would be set explicitly to False, it would practically need to be accompanied with a not_null=True;
• however, when not_null=True the value for when_null_skip= is ignored;
• therefore, this flag meets no true need/use-case for users.

This work includes (by may not be limited to):

• remove any implementation in/around this flag
• remove any mention of this flag in the documentation (on develop, do not re-write history)
• note this conclusion in ytt Proposal :: Other Approaches Considered