tests: remove unused methods from test_s3 and test_swift

Both define get_default_store_specs(), but neither is referenced
anywhere in the codebase.

Change-Id: Ifcf8fb15b5131beb1789c6f0f30b0e9c292592dd
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
Merge "tests: remove unused methods from test_s3 and test_swift"