Return true from Pane#destroyItem if item is destroyed

[kuychaco]

This PR ensures that we return the correct boolean value from Pane.prototype.destroyItem when the force parameter is true or the item should not prompt to save.

Without this, logic such as the following

const destroyed = await pane.destroyItem(item); // returns the value of `item.destroy && item.destroy()` which can be undefined or null
if (!destroyed) {
  pane.activateItem(item); // called on an item that has been destroyed
}

... produces an uncaught error error ...

Uncaught (in promise) Error: Adding a pane item with URI '/some/path/to/item' that has already been destroyed

/cc @maxbrunsfeld for relevant change made in dc32018#diff-1a7d1dda35d8e7ed3ad35b9a9fd2f134L615

NOTE: Fix needs to be cherry-picked to beta on the 1.9-releases branch in order for atom/github CI to pass

[maxbrunsfeld]

👍 Thanks for fixing this @kuychaco!

[kuychaco]

Somehow this got missed when atom/github#980 got merged, and is now superseded by #15264 so I'll go ahead and close it