DRIVERS-2976 Removed deprecated GridFS fields from tests: md5, contentType, aliases

[GromNaN]

Fix DRIVERS-2976

Fields md5, contentType and contentType were deprecated.

• Remove the fields from the fixtures, so we test reading fields that doesn't contain this fields
• Remove the test setting contentType, as this deprecated feature doesn't need to be implemented

From the spec:

If not provided the driver MUST omit the aliases field from the files collection document.

Please complete the following before merging:

• Update changelog.
• Make sure there are generated JSON files from the YAML test files.
• Test changes in at least one language driver. (on PHPLIB)
• Test these changes against all server versions and topologies (including standalone, replica set, sharded clusters, and serverless).