tree: Drop duplicated <stdarg.h> and <stdio.h>

<string.h> is supposed to provide <stdarg.h> and <stdio.h>

Change-Id: I021ba535ba5ec683021c4dfc41ac18d9cebbcfd2
Signed-off-by: Elyes Haouas <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/81853
Reviewed-by: Eric Lai <ericllai@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

tests/lib/string-test.c

@@ -5,7 +5,6 @@
 #include <stdint.h>
 #include <stddef.h>
 #include <tests/test.h>
-#include <stdio.h>
 
 /*
  * Important note: In every particular test, don't use any string-related