Storage async poller should need only one await.

[lmazuel]

Current code will need to "await await" to get the poller result.
This is not the expected syntax, see autorest:
https://github.com/Azure/autorest.python/blob/a251e361218598b55b0621db2275aafcb7158a5c/test/azure/Expected/AcceptanceTests/Lro/lro/aio/operations_async/_lr_os_operations_async.py#L128

Two occurences in Storage;

azure-sdk-for-python/sdk/storage/azure-storage-file/azure/storage/file/aio/directory_client_async.py

Line 304 in e2266eb

return async_poller(

azure-sdk-for-python/sdk/storage/azure-storage-file/azure/storage/file/aio/file_client_async.py

Line 912 in e2266eb

return async_poller(command, start_close, None, polling_method)

[kristapratico]

merged