Removes guide specific to OS X Sierra

What does this Merge Request do?

Removes troubleshooting guide for un-supported OS X version (see !1396 (merged))

Merge Request checklist

• [-] Tests added for new functionality. If not, please raise Issue to follow-up.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• [-] gdk doctor test added, if needed.