Cygwin: drop fixed addresses for standard shared regions

Cygwin: drop fixed addresses for standard shared regions

With the previous commit 9ddd48ee1b8d ("Cygwin: /proc/<PID>/maps:
print real shared region addresses"), the real addresses of
the standard shared regions (cygwin, user, myself, shared console)
are read from the printed process itself.  We don't need fixed
addresses anymore, so drop the definitions and simplify open_shared.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>