WASHINGTON — President Donald Trump’s threat to stop billions of dollars in government payments to insurers and force the collapse of “Obamacare” could put the government in a legal bind.Law experts say he would be handing insurers a solid court case, while undermining his own leverage to compel Democrats to negotiate, especially if premiums jump by 20 percent as expected after such a move.“Trump thinks he’s holding all the cards. But Democrats know what’s in his hand, and he&rsquo …