Remove retrolambda.

[caarmen]

Not sure why we still had it.

It's causing problems now:

A problem occurred configuring root project 'poet-assistant'.
> Could not resolve all artifacts for configuration ':classpath'.
   > Could not find me.tatarka.retrolambda.projectlombok:lombok.ast:0.2.3.a2.
     Searched in the following locations:
       - https://dl.google.com/dl/android/maven2/me/tatarka/retrolambda/projectlombok/lombok.ast/0.2.3.a2/lombok.ast-0.2.3.a2.pom
       - https://jcenter.bintray.com/me/tatarka/retrolambda/projectlombok/lombok.ast/0.2.3.a2/lombok.ast-0.2.3.a2.pom
     Required by:
         project :

[github-actions]

Overall Project	54.02%	❌

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