Remove the EXE file test spec

This thing has been a problem since forever, given cross-platform
issues and random collisions with other file magic signatures.

See rapid7#3807 and rapid7#2793 and the even earlier redmine bug.

This should not close the bug, since we're merely avoiding the random
fails by not testing at all. We /should/ test, but in the meantime, we
can't suffer these failures that throw me (at least) into a panic every
time.

Author: Tod Beardsley