remove duplication of version in __init__ and remove not used variables

probably something like:
`__version__ = importlib.metadata.version('pipreqs')`

refs:
https://docs.python.org/3/library/importlib.html
https://github.com/python-poetry/poetry/issues/273

Start by locating the package __init__ file, the existing version declarations, and the unused variables referenced by the issue. Compare the current metadata setup with importlib.metadata.version and the linked Python documentation; done means one authoritative version source and removal of confirmed unused variables without breaking the package tests or CLI.

Written by the indexing model from the issue text.