fix(sync): Always send firefox.fxaLogout on sign out

[LZoog]

Because:

• If users are signed into the browser but they've accessed account settings in a non-Sync context, the browser holds onto the old session token, leading to a 'Session Expired' page when they try to access CAD or account settings from the browser

This commit:

• Removes the isSync() check around the firefox.fxaLogout call
• Cleans up stuff no longer needed

closes FXA-10770

[vpomerleau]

Confirmed, tested this out locally and it's working as expected! 🏆