remove sbt-whitesource

[xuwei-k]

• Remove Whitesource checks playframework#11003

Remove Whitesource checks as Lightbend has discontinued the use of the Whitesource service.

https://app.travis-ci.com/github/playframework/play-grpc/jobs/557592840#L275

[error] sbtwhitesource.WhiteSourceException: Error communicating with service: Illegal arguments: Bad org token
[error] 	at sbtwhitesource.CheckPoliciesAction.sendCheckPolicies(WhiteSource.scala:385)
[error] 	at sbtwhitesource.CheckPoliciesAction.doExecute(WhiteSource.scala:366)
[error] 	at sbtwhitesource.BaseAction.execute(WhiteSource.scala:76)
[error] 	at sbtwhitesource.WhiteSourcePlugin$.$anonfun$projectSettings$9(WhiteSourcePlugin.scala:152)
[error] 	at sbtwhitesource.WhiteSourcePlugin$.$anonfun$projectSettings$9$adapted(WhiteSourcePlugin.scala:149)

[ihostage]

@mkurz This repo doesn't have a Mergify configuration for auto-merge by label 😞