remove a unused type SecureJSONPrefix

[bestgopher]

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remove an unused type SecureJSONPrefix in render/json.go

[codecov]

Codecov Report

Merging #2391 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2391   +/-   ##
=======================================
  Coverage   98.63%   98.63%           
=======================================
  Files          41       41           
  Lines        2348     2348           
=======================================
  Hits         2316     2316           
  Misses         18       18           
  Partials       14       14           

Impacted Files	Coverage Δ
render/json.go	87.80% <ø> (ø)

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[appleboy]

Please revert this.

[bestgopher]

Done.