Remove the memvfs.c extension, to prevent it from being misused.

Remove the memvfs.c extension, to prevent it from being misused.
See [forum:/forumpost/91811439aa|forum post 91811439aa] for the complaint
that prompted me to take down this demonstration code.  I have other things
to do rather than argue about posts such as the foregoing, so it is easier
to just take the code down.  If you need this code, you can downloading older
check-ins.  The beauty of DAG-based version control systems like Fossil
(and Git) is that nothing is ever lost; it is just stops appearing on
trunk.

FossilOrigin-Name: eb79110dcac80dde3bcca0929d67a8f8f6d2eea5695184b2b7c6788283f06902