Remove ciphers/prehistoric_men.txt

[rohanr18]

Describe your change:

No reason to keep this file

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Documentation change?

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doctests that pass the automated testing.
• All new algorithms include at least one URL that points to Wikipedia or another similar explanation.
• If this pull request resolves one or more open issues then the description above includes the issue number(s) with a closing keyword: "Fixes #ISSUE-NUMBER".

[tianyizheng02]

The file is used in ciphers/transposition_cipher_encrypt_decrypt_file.py as a demo file to encrypt/decrypt. However, the code in transposition_cipher_encrypt_decrypt_file.py has the wrong file name, so it wouldn't find the file anyway. In either case, it's not entirely useless.

[rohanr18]

Alright, will make a change to transposition_cipher_encrypt_decrypt_file.py then

[rohanr18]

Is it better to make a folder, because we have 3 files for transposition cipher?