Remove unused performance testing packages

CHANGELOG.md

cmd/loadtest/main.go

tests/performance/.gitignore

...e_reports/1561342783121-BeforeSingleflight-ColdCache.json

...e_reports/1561342955384-BeforeSingleflight-WarmCache.json

...le_reports/1561432114806-AfterSingleflight-ColdCache.json

...le_reports/1561432138475-AfterSingleflight-WarmCache.json

...orts/1561514534005-AfterCustomSingleflight-ColdCache.json

...orts/1561514563170-AfterCustomSingleflight-WarmCache.json

...ance/example_reports/1584652894896-Checkup-ColdCache.json

...ance/example_reports/1584652946355-Checkup-WarmCache.json

...ils/t2bot.io_00845bfa7e453d392f86b53eadbdbfd3d10d3fcd.png

...ils/t2bot.io_0850052a28cf72d91c77b52b74e67cc9fccd3607.png

...ils/t2bot.io_102d17f15f25e697d857c5d5003efcf27c36886f.png

...ils/t2bot.io_13754a79523fb6c96abdb06a52fd2b54788d1a2a.png

...ils/t2bot.io_144c17562179ad788652961dd5238e8796f6a0a5.png

...ils/t2bot.io_180a386b83717f75785c4f0501ae1551ff448a1b.png

...ils/t2bot.io_1e4925f241581d27d5dfff7d7b0bfaf62714b4c3.png

...ils/t2bot.io_20777c9ccca38567559ef08764602ee5c2ab6927.png

...ils/t2bot.io_3fab93bdf0643c191721426aa930aab9aa61fc47.png