fix(build): remove clean task to pass on Windows

[lemeurherve]

This PR removes the clean gradle task not needed on ci.jenkins.io ephemeral agents to fix the following Windows builds error:

10:22:17  FAILURE: Build failed with an exception.
10:22:17  
10:22:17  * What went wrong:
10:22:17  Execution failed for task ':clean'.
10:22:17  > java.io.IOException: Unable to delete directory 'C:\Jenkins\agent\workspace\ugins_idea-stapler-plugin_PR-160\build'
10:22:17      Failed to delete some children. This might happen because a process has files open or has its working directory set in the target directory.
10:22:17      - C:\Jenkins\agent\workspace\ugins_idea-stapler-plugin_PR-160\build\tmp\.cache\expanded\expanded.lock
10:22:17      - C:\Jenkins\agent\workspace\ugins_idea-stapler-plugin_PR-160\build\tmp\.cache\expanded
10:22:17      - C:\Jenkins\agent\workspace\ugins_idea-stapler-plugin_PR-160\build\tmp\.cache
10:22:17      - C:\Jenkins\agent\workspace\ugins_idea-stapler-plugin_PR-160\build\tmp

Ref: jenkins-infra/helpdesk#3744

Testing done

Tested via a replay:

• https://ci.jenkins.io/job/Plugins/job/idea-stapler-plugin/job/PR-160/3/console
• https://ci.jenkins.io/job/Plugins/job/idea-stapler-plugin/job/PR-163/2/console

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[duemir]

OK. If clean is redundant due to ephemeral agents, let us remove it.