Revert ad15c2, which causes Windows seg-faults (Trac #8834)

We don't yet understand WHY commit ad15c2, which is to do with
CmmSink, causes seg-faults on Windows, but it certainly seems to.  So
reverting it is a stop-gap, but we need to un-block the 7.8 release.

Many thanks to awson for identifying the offending commit.