truncate: Remove unnecessary includes

These are no longer necessary, as we are now using LibCore syscall
wrappers.

Userland/Utilities/truncate.cpp

@@ -7,10 +7,6 @@
 #include <LibCore/ArgsParser.h>
 #include <LibCore/System.h>
 #include <LibMain/Main.h>
-#include <fcntl.h>
-#include <stdio.h>
-#include <sys/stat.h>
-#include <unistd.h>
 
 enum TruncateOperation {
     OP_Set,