Remove stale Build Checks feature

[gp201]

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We're no longer planning to use the staleBuildChecks feature.

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1. Remove https://github.com/oppia/oppiabot/blob/master/lib/staleBuildChecks.js and the associated unit tests
2. Remove

   oppiabot/index.js

   Line 152 in fcc2aab

   staleBuildModule.checkAndTagPRsWithOldBuilds(context),

3. Remove from constants.js

   oppiabot/constants.js

   Line 60 in fcc2aab

   const staleBuildLabelCheck = 'stale-build-label-check';

   and

   oppiabot/constants.js

   Line 114 in fcc2aab

   staleBuildLabelCheck],

   and

   oppiabot/constants.js

   Line 179 in fcc2aab

   module.exports.staleBuildLabelCheck = staleBuildLabelCheck;

[gp201]

Duplicate of #324