pp.c: Add comment
Experimentally cause 'use encoding' to fail.

This actually doesn't remove it, but causes any program that does use it
to die.  Its use has raised a (default-on) deprecation warning since
5.18, but its demise was omitted from the perldelta for v5.20.

This commit does the minimum necessary to flush out what might break by
its actual removal, while being easily revertable should that become
necessary.