Clean up a live hack from wmf-deployment r53208: option to short-circuit math path...

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;