Remove unused message 'livepreview-loading'

Follow-up to 53bd3da58fc5c3f. The message has been added to
SemanticMediaWiki in change I14448aecbbef2cfcfb5f47bd882046f0cc3c3ad1

Change-Id: I76d10534c64d3308423611a8288f7f7335cff1d3
Merge "Remove unused message 'livepreview-loading'"