Clean up a live hack from wmf-deployment r53208: option to short-circuit math path and file existence checks.
Adds $wgMathCheckFiles setting:

/**

• Normally when generating math images, we double-check that the
• directories we want to write to exist, and that files that have
• been generated still exist when we need to bring them up again. *
• This lets us give useful error messages in case of permission
• problems, and automatically rebuild images that have been lost. *
• On a big site with heavy NFS traffic this can be slow and flaky,
• so sometimes we want to short-circuit it by setting this to false. */

$wgMathCheckFiles = true;