Deduping of new variable definitions when delegating to schemas

[adamkl]

This PR provided a fix for issue #602 where duplicate variables were being generated when delegating to a schema with multiple fragments referencing a linked field.

Added minor change to remove duplicates from the newVariableDefinitions created in delegateToSchema.ts. All exiting tests pass, and the new tests create do demonstrate the issue now pass as well.

TODO:

• If this PR is a new feature, reference an issue where a consensus about the design was reached (not necessary for small changes)
• Make sure all of the significant new logic is covered by tests
• Rebase your changes on master so that they can be merged easily
• Make sure all tests and linter rules pass
• Update CHANGELOG.md with your change

[apollo-cla]

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[renatobenks-zz]

sounds nice fix!