fix: do not spam the log with checksum related INFO messages when downloading using transfer_manager

[rafalh]

download_chunks_concurrently function does not allow to set checksum field in download_kwargs. It also does not set it on its own so it takes the default value of "md5" (see Blob._prep_and_do_download). Because ranged downloads do not return checksums it results in a lot of INFO messages (tens/hundreds):

INFO google.resumable_media._helpers - No MD5 checksum was returned from the service while downloading ... (which happens for composite objects), so client-side content integrity checking is not being performed. 

To fix it set the checksum field to None which means no checksum checking for individual chunks. Note that transfer_manager has its own checksum checking logic (enabled by crc32c_checksum argument)

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Fixes #1358 🦕

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[cojenco]

I think this is a reasonable patch to avoid passing in the default checksum md5 in the subsequent call blob._prep_and_do_download. @andrewsg thoughts/concerns to this change?

@rafalh Could you update the corresponding unit tests in https://github.com/googleapis/python-storage/blob/main/tests/unit/test_transfer_manager.py?

[andrewsg]

This makes sense to me. Thank you for your submission!

[rafalh]

I fixed the tests and got them to pass locally (executed only tests/unit/test_transfer_manager.py). I had to make a change to download_chunks_concurrently so it makes a copy of download_kwargs dict argument before changing it (which is a good approach anyway). Otherwise some tests changed DOWNLOAD_KWARGS global variable and made other tests fail
Also removed unused expected_download_kwargs variables in some tests

[cojenco]

LGTM, thank you for working on this!