Clean up XHP region properties

[Wilfred]

Currently we set text properties but never remove them, resulting in modified code incorrectly being considered XHP.

[Wilfred]

As of c2c032e, a workaround is to restart hack-mode (i.e. M-x hack-mode) when this occurs.

[Wilfred]

I'm struggling to replicate this. Will reopen if I see it happen again.

[Wilfred]

OK, I can replicate.

// This will run, but it's not doing what the user expects. We never
// pass arguments to entry points.
<<__EntryPoint>>
function bad_variadic(mixed ...$_): void {}

Insert this into a file. Next, start typing a <?hh header before hand. Observe that the highlighting of __EntryPoint breaks.