Prune non-go files from vendoring

[mboersma]

What this PR does / why we need it:

Removes 218 unnecessary files from the vendor/ directory.

See https://github.com/golang/dep/blob/master/docs/Gopkg.toml.md#prune for information about pruning vendored files with dep.

Special notes for your reviewer:

If applicable:

• documentation
• unit tests
• tested backward compatibility (ie. deploy with previous version, upgrade with this branch)

Release note:

Prune non-go files from vendoring

[tariq1890]

/lgtm

[acs-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: mboersma, tariq1890

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