In case this is news to you, a president can't repeal an existing law with an executive order. Some (looking at you, Barack Obama) have to tried to enact de facto repeals by issuing orders refusing to enforce laws in whole or in part, but the law stays on the books when that happens.
So ObamaCare is still the law because John McCain, Susan Collins, Lisa Murkowski and Rand Paul are frauds. That being the case, Donald Trump made a move today to make the law loss costly, which is ironic considering its actual, formal name. Is it legal to do what Trump did? That's an interesting question, because ObamaCare mandates that insurance policies cover certain things, regardless of whether consumers want to pay for these things to be covered, and doesn't give consumers or insurers the option of negotiating plans that don't cover the mandated benefits.
