remove unused file

src/passthrough_gtk4.c

@@ -1,12 +0,0 @@
-#include <stdio.h>
-
-#define PROJECT_NAME "passthrough-gtk4"
-
-int main(int argc, char **argv) {
-    if(argc != 1) {
-        printf("%s takes no arguments.\n", argv[0]);
-        return 1;
-    }
-    printf("This is project %s.\n", PROJECT_NAME);
-    return 0;
-}