Remove unused functions from ArtifactCache

Background

The functions pull_tree(), push_directory(), and push_message() in ArtifactCache were used by the remote execution system and will no longer be needed for BuildStream once !946 (merged) is merged. However, some of the tests still use them, so they cannot be removed until the tests are converted.

Task description

• Convert the tests which use pull_tree(), push_directory(), or push_message() to be tests of the CASCache object instead of the artifact cache, and supply manually created remotes to test them.
• Remove the old functions from ArtifactCache.

Acceptance Criteria

All tests continue to pass, previous behaviour is still tested, and the mentioned functions are removed from ArtifactCache.

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