Removed exclusion of files that end in .java (fixing empty sources.jar)

[langsmith]

Resolves #1053 and undos be2f75c#diff-c197962302397baf3a4cc36463dce5ea

cc @gwimmel, who originally reported #1053

[tobrun]

this doesn't solve the initial issue of #989

[gwimmel]

maybe something like the following could be used instead?

  tasks.withType(Jar) { jarTask ->
    if (!jarTask.name.endsWith("SourcesJar")) {
      jarTask.exclude("**/*.java")
    }
  }

[tobrun]

fixed with #1064