How to remove ground pin on power cable

Just snap it off with needle nose pliers

While it's not likely to get a shock from a sub, if it was grounded, I would try and keep it that way. The ground loop is most likely from cable TV. Unscrew it and see. Find a way to break the ground there. Jensen Transformers is a good solution.https://www.jensen-transformers.com/home-theater/video/

Yikes if you insist on the removal of the ground pin at least install a GFCI receptacle in the place of the standard outlet. In the event your sub has an issue it will trip before any damage to person or property. 
Some of you do not have the understanding of how an electrical system functions. Stick with what you know. It is dangerous to advise on thing that can cause damage to people and/or property. 
Remember true intelligence is knowing when not to advise. 

i like Dills   response 

Yikes if you insist on the removal of the ground pin at least install a GFCI receptacle in the place of the standard outlet. In the event your sub has an issue it will trip before any damage to person or property.

"According to a 1999 study by the American Society of Home Inspectors, 21% of GFCI circuit breakers and 19% of GFCI receptacles inspected didn’t provide protection, leaving the energized circuit unprotected. In most cases, damage to the internal transient voltage surge protectors (metal-oxide varistors) that protect the GFCI sensing circuit were responsible for the failures of the protection devices. In areas of high lightning activity, such as southwest Florida, the failure rate for GFCI circuit breakers and receptacles was over 50%!"
See https://www.ecmweb.com/basics/article/20901772/how-gfcis-work

A GFCI will only help if the CBLF is grounded and not contacting the neutral. A GFCI will not trip if the CBLF is between Hot and Neutral.

If you have GFCI, test them monthly and after any electrical event, preferably with an external tester.