Removed duplicate declaration of mockito-core in rocksdb-checkpoint-differ

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[WARNING] Some problems were encountered while building the effective model for org.apache.ozone:rocksdb-checkpoint-differ:jar:1.4.0-SNAPSHOT
[WARNING] 'dependencies.dependency.(groupId:artifactId:type:classifier)' must be unique: org.mockito:mockito-core:jar -> duplicate declaration of version (?) @ line 85, column 17
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[WARNING] It is highly recommended to fix these problems because they threaten the stability of your build.
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[WARNING] For this reason, future Maven versions might no longer support building such malformed projects.
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