Rely on C++17 <new> in new-op.cc

gdbsupport/new-op.cc has a comment about relying on the C++-17 <new>
header.  This patch implements the suggestion.

Approved-By: Pedro Alves <pedro@palves.net>

gdbsupport/new-op.cc

diff --git a/gdbsupport/new-op.cc b/gdbsupport/new-op.cc
index 6a056ef..181fc49 100644
--- a/gdbsupport/new-op.cc
+++ b/gdbsupport/new-op.cc
@@ -27,11 +27,6 @@
 #include "host-defs.h"
 #include <new>
 
-/* These are declared in <new> starting C++14.  Add these here to enable
-   compilation using C++11. */
-extern void operator delete (void *p, std::size_t) noexcept;
-extern void operator delete[] (void *p, std::size_t) noexcept;
-
 /* Override operator new / operator new[], in order to internal_error
    on allocation failure and thus query the user for abort/core
    dump/continue, just like xmalloc does.  We don't do this from a