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 #3807 and #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.
Remove the helper as well.

See #3807.

Again, happy to have working tests that actually test validity. But,
these ain't them.
Land #4420, tentative "fix" for EXE spec