Well, I am not a lawyer but the situation seems pretty clear. You are not the original owner but you can still use the code under the MIT license, simply meaning that you have to adhere to its license. Given that MIT license is so permissive it basically just means that you should not remove the license information. That said, as others are trying to point out, there may be other issues that are NOT related to the co…
There are probably whole countries that forbid these contract provisions, or forbid enforcing them.
Don't assume that any particular right you signed away to get a job is really gone. Ask a local lawyer. Expect to be surprised. In many cases, even when there is no specific law, case law has interpreted such clauses very restrictively, making them de facto unenforceable for, e.g., anybody below company officer level.