Remove testing "update()" with "version" only

We have tests which make sure we can use `Timestamp.update()` and
`Snapshot.update()` with MetaFile instance storing only version
(because length and hashes are optional).
Those tests were created to make sure that we are actually supporting
optional hashes and length when we call `update` for those classes, but
after we changed the `update()` signature to accept `MetaFile` instance
the tests are obsolete.
The reason is that length and hashes can be optional because of the
MetaFile implementation, no the update function itself and we have
other tests validating creating a MetaFie instance without hashes and
length.

Signed-off-by: Martin Vrachev <mvrachev@vmware.com>

Author: MVrachev

tests/test_api.py

@@ -306,13 +306,6 @@ def test_metadata_snapshot(self):
             snapshot.signed.meta['role1.json'].to_dict(), fileinfo.to_dict()
         )
 
-        # Update only version. Length and hashes are optional.
-        fileinfo = MetaFile(3)
-        snapshot.signed.update('role2', fileinfo)
-        self.assertEqual(
-            snapshot.signed.meta['role2.json'].to_dict(), fileinfo.to_dict()
-        )
-
         # Test from_dict and to_dict without hashes and length.
         snapshot_dict = snapshot.to_dict()
         del snapshot_dict['signed']['meta']['role1.json']['length']
@@ -366,13 +359,6 @@ def test_metadata_timestamp(self):
         timestamp_test = Timestamp.from_dict(test_dict)
         self.assertEqual(timestamp_dict['signed'], timestamp_test.to_dict())
 
-        # Update only version. Length and hashes are optional.
-        fileinfo = MetaFile(version=3)
-        timestamp.signed.update(fileinfo)
-        self.assertEqual(
-            timestamp.signed.meta['snapshot.json'].to_dict(), fileinfo.to_dict()
-        )
-
     def test_key_class(self):
         keys = {
             "59a4df8af818e9ed7abe0764c0b47b4240952aa0d179b5b78346c470ac30278d":{