build: fix directories removal

The 'dirs' array contains a single-level listing of a directory, e. g.
['app', 'build', 'build.gradle', 'gradle', '.gradle']. Multi-component
paths like 'build/tmp' could never be found in this array and thus were
never removed.

Call shutil.rmtree() without checking that the argument is in 'dirs'. If
it exists and is a directory, it'll be removed. Otherwise shutil.rmtree()
will do nothing.
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f