Remove obsolete head field from plugin manifest

[corneliusweig]

Close #152

Once a new krew version with this PR is released, the head fields in all krew-index plugin yamls can be removed. Luckily, no plugin exclusively relies on the head field and all manifests also have the url set.

[codecov-io]

Codecov Report

Merging #212 into master will increase coverage by 0.57%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #212      +/-   ##
==========================================
+ Coverage   53.77%   54.35%   +0.57%     
==========================================
  Files          13       13              
  Lines         716      701      -15     
==========================================
- Hits          385      381       -4     
+ Misses        279      269      -10     
+ Partials       52       51       -1

Impacted Files	Coverage Δ
pkg/installation/upgrade.go	0% <0%> (ø)	⬆️
pkg/installation/install.go	36.36% <0%> (+1.19%)	⬆️
pkg/installation/util.go	60.81% <100%> (-0.44%)	⬇️
pkg/index/validate.go	94.44% <100%> (ø)	⬆️

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[ahmetb]

Looks excellent, thanks.
/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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