Clean-up `samples/snippets/requirements.txt`

[tswast]

Ideally we'd have some code samples which only require the bare minimum dependencies (no pandas, no google-cloud-bigquery-storage, no protobuf, etc). The samples/snippets directory would be perfect for this, but:

1. There are a couple of pandas-related samples there which ideally should be moved to a samples/pandas directory.
2. Likewise, there is a "desktop application" sample https://github.com/googleapis/python-bigquery/blob/main/samples/snippets/user_credentials.py which ideally belongs in its own directory with its own noxfile.py and requirements.txt. This is because it's responsible for the google-auth-oauthlib dependency, which isn't required for any other samples. Maybe we call this directory samples/desktopapp or something?

Related issues:

• There are still code samples that haven't been moved to subdirectories: standardize samples directory layout #790

[chalmerlowe]

Added this content to another open ticket #790
Closing this since that ticket now captures the relevant information and this is redundant.