chore: remove unused .txt files (#13842)

Some packages have an `.eggs` directory with a README.txt. This file
doesn't add value and is unused.


https://github.com/googleapis/google-cloud-python/tree/main/packages/google-cloud-bigquery-connection/.eggs

The `.eggs` directory is ignored however these files may have been added
prior to this `.gitignore` change.

https://github.com/googleapis/google-cloud-python/blob/3c829a0e14848fd084ceb1fcdaf5c5336e261df8/.gitignore#L13

I also removed
`packages/google-cloud-common/google-cloud-common/google-cloud-common.txt`
which is not used.

Author: parthea