Removed Unused `import sys` (#922)

I removed `import sys` because it is not used in the program.
This addresses a [recommendation from lgtm](https://lgtm.com/projects/g/TheAlgorithms/Python/snapshot/66c4afbd0f28f9989f35ddbeb5c9263390c5d192/files/ciphers/caesar_cipher.py?sort=name&dir=ASC&mode=heatmap)