tests,cputest: remove struct.c

* This test will introduce `ret_imm` instruction in x86.
  Except that, it does not test other things.

tests/cputest/tests/struct.c

@@ -1,30 +0,0 @@
-#include "trap.h"
-
-#define N 40
-
-struct dummy {
-	int pad1[N];
-	char pad2[N];
-} d;
-
-__attribute__((noinline))
-struct dummy fun(struct dummy a) {
-	return a;
-}
-
-int main() {
-	int i;
-	for(i = 0; i < N; i ++) {
-		d.pad1[i] = i + 128;
-		d.pad2[i] = i;
-	}
-
-	struct dummy t = fun(d);
-
-	for(i = 0; i < N; i ++) {
-		nemu_assert(t.pad1[i] == i + 128);
-		nemu_assert(t.pad2[i] == i);
-	}
-
-	return 0;
-}