LibJS: Remove unused LibCore/DateTime.h header from Date.cpp

We use a double for [[DateValue]] and the spec's own AOs for any
calculations now.

Userland/Libraries/LibJS/Runtime/Date.cpp

@@ -8,7 +8,6 @@
 #include <AK/NumericLimits.h>
 #include <AK/StringBuilder.h>
 #include <AK/Time.h>
-#include <LibCore/DateTime.h>
 #include <LibJS/Runtime/AbstractOperations.h>
 #include <LibJS/Runtime/Date.h>
 #include <LibJS/Runtime/GlobalObject.h>