Cleanup duplicated "credentials can not sign" error

[theacodes]

Context: #2998 (comment)

[lukesneeringer]

@jonparrott Is this still a thing?

[theacodes]

Good question - @dhermes is it?

[dhermes]

We still have duplicated code:

$ git log -1 --pretty=%H
d750a1394781dbe33679057c02233d2dfc2540e1
$ git grep -n 'you need a private key to sign'
storage/google/cloud/storage/_signing.py:51:        raise AttributeError('you need a private key to sign credentials.'
storage/google/cloud/storage/bucket.py:1121:                'you need a private key to sign credentials.'

Now that google.cloud.storage._signing exists, you could probably move the check from Bucket.generate_upload_policy() into the _signing module.

[lukesneeringer]

Meta-comment: This is a really good example of an issue where a TODO in the code would have served a lot better than an issue does. The issue actually loses context.