Remove screenshot as a system command for the command palette

While it's kinda cool... it's not really very helpful if you're doing things
via textual-web; all you're going to do is start to use storage on the host
machine, not the client machine (unless they're the same thing, of course).

src/textual/_system_commands_source.py

@@ -34,11 +34,6 @@ class SystemCommandSource(CommandSource):
                 self.app.action_toggle_dark,
                 "Toggle the application between light and dark mode",
             ),
-            (
-                "Save a screenshot",
-                self.app.action_screenshot,
-                "Save a SVG file to storage that contains the contents of the current screen",
-            ),
             (
                 "Quit the application",
                 self.app.action_quit,