Decrypted string is logged in plain text when log level is DEBUG

**Is your feature request related to a problem? Please describe.**
Decrypted secret is logged when log level is DEBUG, meaning it can easily end up in CI/CD pipeline logs or show up when screen sharing.

**Describe the solution you'd like**
Remove [this](https://github.com/vsajip/python-gnupg/blob/efb2043730b6db890daa20344eea4cd8592c8de6/gnupg.py#L2200) line.
