Remove unused setting $wgImageMagickIdentifyCommand

No need to deprecate first, as it is simply unused.

It was added in r107351, and its only usage was removed in
r108141.

Change-Id: Ie95d8b707ac1ff2fe2a9bae6d5f77ba68b4291ad
Merge "Remove unused setting $wgImageMagickIdentifyCommand"