Fix: remove already-defined export

[alexkrolick]

Fixes #3910:

Babel 6 errors when overwriting a named export that's already been defined, including by export * from '...'

Checklist:

• If this PR is a new feature, please 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

[apollo-cla]

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[benjamn]

Thanks @alexkrolick!