Remove outdated version.txt file

[jorgepiloto]

🐞 Problem

The version.txt file was introduced in df5cbc9, at the very beginning of the project. The setup.py from those days pointed at this file to extract the version of pyfluent.

Nowadays, the version of the project is specified in the ansys/fluent/core/_version.py file, replacing the purpose of the version.txt file.

💡 Solution

Remove the version.txt file as it does not contain any information and is no longer required.