[kernel] Remove placement-new declaration from memory.h.cog

Finishing the trend of using `#include <new>` to define new, get rid of
the last bits of custom-declared operator new.

src/kernel/device_manager.cpp

@@ -1,3 +1,4 @@
+#include <new>
 #include <stddef.h>
 #include <stdint.h>
 #include <util/misc.h> // for checksum

src/kernel/heap_allocator.cpp

@@ -1,3 +1,4 @@
+#include <new>
 #include <stdint.h>
 #include <string.h>
 

src/kernel/logger.cpp

@@ -1,3 +1,4 @@
+#include <new>
 #include <string.h>
 
 #include <util/no_construct.h>

src/kernel/memory.h.cog

@@ -13,8 +13,6 @@ namespace bootproto {
     struct args;
 }
 
-void * operator new (size_t, void *p) noexcept;
-
 /// Allocate from the default allocator.
 /// \arg size  The size in bytes requested
 /// \returns   A pointer to the newly allocated memory,