Removed duplicate messages from diagnosticMessages.json.
Removed unused diagnostics.
Removed more unused diagnostics.
Recovered deleted, renumbered existing, reworded some.
Addressed CR feedback, removed NoPrefix messages.
Merge branch 'master' into diagnosticsCleanup
Removed duplicate diagnostic.
Merge branch 'master' into diagnosticsCleanup
Reassigned diagnostic codes.
Update LKG
Merge branch 'master' into diagnosticsCleanup
Merge pull request #383 from Microsoft/diagnosticsCleanup

Cleaned up diagnostic messages.