Remove duplicate Collatz sequence implementation

[tianyizheng02]

Feature description

I think one of 3n_plus_1.py and collatz_sequence.py should be removed, as they're nearly identical implementations of the same algorithm for computing Collatz sequences.

I'm not sure which of the two files should be deleted. I believe that "the Collatz conjecture" is the more well-known name for the problem, but it seems that 3n_plus_1.py is the older of the two files.

[Nomine-Nihil]

This I didn't see.. V C But I'm having trouble running and recognizing.. ??