Remove support for unboxed doubles

As of the mentioned upstream commit, the hack we use to detect the
possibility of unboxed doubles no longer works. Rather than trying to
create another hack, simply hardcode it to false. This requires us to
also remove support for EOL versions of Node.js which may still have
unboxed doubles.

.github/workflows/push.yml

@@ -16,8 +16,6 @@ jobs:
       fail-fast: false
       matrix:
         node:
-          - version: 10.x
-          - version: 12.x
           - version: 14.x
           - version: 16.x
           - version: 18.x

src/llv8-constants.cc

@@ -139,16 +139,10 @@ void Map::Load() {
 
 
 bool Map::HasUnboxedDoubleFields() {
-  // LayoutDescriptor is used by V8 to define which fields are not tagged
-  // (unboxed). In preparation for pointer compressions, V8 disabled unboxed
-  // doubles everywhere, which means Map doesn't have a layout_descriptor
-  // field, but because of how gen-postmortem-metadata works and how Torque
-  // generates the offsets file, we get a constant for it anyway. In the future
-  // unboxing will be enabled again, in which case this field will be used.
-  // Until then, we use the presence of this field as version (if the field is
-  // present, it's safe to assume we're on V8 8.1+, at least on supported
-  // release lines).
-  return !kLayoutDescriptor.Loaded();
+  // V8 has now disabled unboxed doubles in all supported Node.js branches. Per
+  // the V8 authors (v8/v8@42409a2e) it seems unlikely this support will ever
+  // return, so we could probably just remove it entirely.
+  return false;
 }
 
 void JSObject::Load() {