RFR: 2168: Don't preserve invalid local clone

[Zhao Song]

When skara bot is running, it frequently clones a lot of repos to the local machine and sometimes, the clone would be invalid due to various reasons.

Currently, if the skara bot found an invalid local clone, it will preserve the invalid clone by renaming it. ErikJ said the original authors might want to do some investigations on the invalid clones, so they chose to preserve the invalid clone.

But now, we rarely investigate the clones. And as time goes by, the local machine running skara will have more and more invalid clones and the disk usage will be very high. And skara admin will need to clean the invalid repos periodically.

To fix this issue, we should just clear the invalid repo in HostedRepositoryPool#removeOldClone.

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Commit messages:
 - SKARA-2168

Changes: https://git.openjdk.org/skara/pull/1615/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1615&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2168
  Stats: 13 lines in 1 file changed: 0 ins; 10 del; 3 mod
  Patch: https://git.openjdk.org/skara/pull/1615.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1615/head:pull/1615

PR: https://git.openjdk.org/skara/pull/1615