Drop build system patches converting Gradle+Kotlin to Gradle+Groovy

Current version of Gradle in Debian is 4.4.x. This version of Gradle can't be used by a project for a build system written in Kotlin (build.gradle.kts files). For that we need Gradle 6.x version, which is not available in Debian. This is due to bootstrap problem of latest Gradle needing Kotlin and Kotlin needing Gradle to build.

To solve this bootstrap problem Kotlin's build system was converted from Gradle+Kotlin to Gradle+Groovy. This is done as a series of patches kept in debian/patches. Updating to newer upstream version and maintaining these patches is hard. So, drop these patches and use Gradle+Kotlin build system.

This can only be done after Gradle version 6.x supporting Gradle+Kotlin is available in Debian. Kotlin is about to be uploaded in to debian/unstable, updating gradle version is next step.