Remove @deprecated from wfTimestamp()

Remove @deprecated from wfTimestamp()

Adding abbreviated wrappers for lengthy function invocations is good
practice, it improves code readability. If all we ever had was
MWTimestamp, and we had a million instances of

$timestamp = new MWTimestamp( $ts );
$mwTimestamp = $timestamp->getTimestamp( TS_MW );

I would introduce a global function or class static method wrapper to
clean up those invocations. But I don't have to, because we already have
the wrapper.

Change-Id: I9149a7626676a0115a0a877f27d8b65e6e868de9