runlisp.c: Remove long-removed `-C' option from usage synopsis.

runlisp.c: Remove long-removed `-C' option from usage synopsis.

This dates back to an earlier time when all configuration was done using
command-line options.  The `-P' option would add one or more Lisp
implementation names to the preferred list; but, because `runlisp'
would collect options from multiple places, it was necessary to be able
to clear the list.  This is what the `-C' option did.  It was removed
as soon as the new configuration-file system was introduced -- which
happened about a week after the initial version was committed.  But
somehow the `-C' option lived on in the usage synopsis.