[cleanup] Remove obsolete Map::FixProxy().

The implementation of the method is long gone, just the declaration was
left for some reason.

Bug: v8:9183
Change-Id: Id68fc6afa63c3663769f9d8e0a6bc1a7a3aa5e35
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1601497
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Auto-Submit: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61361}

src/objects/map.h

@@ -727,8 +727,6 @@ class Map : public HeapObject {
       Handle<Symbol> transition_marker, const char* reason,
       bool support_holey_frozen_or_sealed_elements_kind = true);
 
-  static Handle<Map> FixProxy(Handle<Map> map, InstanceType type, int size);
-
   // Maximal number of fast properties. Used to restrict the number of map
   // transitions to avoid an explosion in the number of maps for objects used as
   // dictionaries.