Remove redundant xclip -f flag because we pipe stdout to null now

See https://gitlab.com/timvisee/ffsend/issues/93

src/util.rs

@@ -395,7 +395,6 @@ impl ClipboardType {
         Self::sys_cmd_set(
             "xclip",
             Command::new(path.unwrap_or_else(|| "xclip".into()))
-                .arg("-f")
                 .arg("-sel")
                 .arg("clip"),
             content,