rune/libenclave: Remote unused pal.h in skeleton

Signed-off-by: Jia Zhang <zhang.jia@linux.alibaba.com>

rune/libenclave/internal/runtime/pal/skeleton/pal.h

-int pal_skeleton_init(char *instance_path);
-int pal_skeleton_exec(char *path, char *argv[], int *exit_value,
-			int stdin_fd, int stdout_fd, int stderr_fd);
-int pal_skeleton_kill(int sig, int pid);
-int pal_skeleton_destroy();