Remove license header from krew.yaml

[ahmetb]

Consistency with other plugin manifests at krew-index repository.

[corneliusweig]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ahmetb, corneliusweig

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[codecov-io]

Codecov Report

Merging #416 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #416   +/-   ##
=======================================
  Coverage   56.41%   56.41%           
=======================================
  Files          19       19           
  Lines         927      927           
=======================================
  Hits          523      523           
  Misses        349      349           
  Partials       55       55

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