Remove spurious -shareexists reference in Plink docs.

That option does exist, but only on master; it was not in the 0.66
release. It turned up by mistake when I updated the documentation copy
of the Plink online help while preparing the 0.66 release, because I
ran plink from the wrong branch.

The new release automation should stop that kind of mistake from
happening in future.

doc/plink.but

@@ -80,8 +80,6 @@ use Plink:
 \c   -N        don't start a shell/command (SSH-2 only)
 \c   -nc host:port
 \c             open tunnel in place of session (SSH-2 only)
-\c   -shareexists
-\c             test whether a connection-sharing upstream exists
 
 Once this works, you are ready to use Plink.