deps: remove duplicated postmortem metadata entry

Signed-off-by: Matheus Marchini <mmarchini@netflix.com>

PR-URL: nodejs#32521
Refs: v8/v8@8.1.307.20...8.1.307.26
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

common.gypi

@@ -35,7 +35,7 @@
 
     # Reset this number to 0 on major V8 upgrades.
     # Increment by one for each non-official patch applied to deps/v8.
-    'v8_embedder_string': '-node.11',
+    'v8_embedder_string': '-node.12',
 
     ##### V8 defaults for Node.js #####
 

deps/v8/tools/gen-postmortem-metadata.py

@@ -280,7 +280,6 @@
     'SlicedString, offset, SMI, kOffsetOffset',
     'ThinString, actual, String, kActualOffset',
     'Symbol, name, Object, kDescriptionOffset',
-    'Map, constructor_or_backpointer, Object, kConstructorOrBackPointerOrNativeContextOffset',
 ];
 
 #