Remove 4 unused live preview messages

Most of these are obsolete because there is no longer a separate
"Live preview" button on the edit form.

* showlivepreview
* livepreview-ready
* livepreview-failed
* livepreview-error

SemanticMediaWiki uses 'livepreview-loading', so I left that message
in for now.

Change-Id: I51d014eb2c3dcf5ff1b91a00d5dc3e5b0c580459
Merge "Remove 4 unused live preview messages"