Completely remove settings panel when uninstalling a package

Fixes https://github.com/atom/settings-view/issues/72.

After poking around for a bit, this seems to resolve the settings panel problem for reinstalled packages. From what I could tell, the old settings panel for the package was not completely removed when the package was uninstalled. As a result, when the package was re-installed -- the old panel was still being used (instead of the new one), and that caused Atom to crash and other problems described in https://github.com/atom/settings-view/issues/72.