Eliminate kernel warning when running tests on FreeBSD

pexpect had a hack to work around some old buggy platforms, and as a
result of the hack running the tests on FreeBSD produced a stream of
kernel warnings in the system log:

  Feb 5 17:19:11 feynman kernel: WARNING pid 11323 (python2.7):
    ioctl sign-extension ioctl ffffffff80087467

The hack has now been removed upstream, so remove it here too.

llvm.org/pr18749