fix: revert window close buffer cleanup code

[cseickel]

fixes #1415

This reverts the renderer close function to ensure that the buffer is deleted synchronously instead of being scheduled for later. That code was originally changed as part of trying to support winfixbuf. I have confirmed that the winfixbuf situation still works correctly even with this reversion, so the change was never actually needed.

See #1418 (comment) for more background.

The one known bug that was fixed by the prior code was when you have neotree as the only open window, then open a float neotree and try to open a file. That's an odd edge case and not worth complicating the code base for. If someone can find a simple and obvious fix for that then we can address it separately.