chore(gradle): remove custom startScripts task

[xeruf]

Reopening of https://github.com/software-challenge/backend/pull/369

Current issues:

• [ ] the start-script hardcodes classpath to lib: https://github.com/gradle/gradle/issues/7033
• [ ] script assumes it is in a bin directory, but I want it in root ("pwd -P")

https://docs.gradle.org/current/dsl/org.gradle.jvm.application.tasks.CreateStartScripts.html

May fix #265