compiler: remove unused Type::in_heap_ member variable

This member variable was added in https://golang.org/cl/46490, but it
was never used. The code uses Named_type::in_heap_ instead.

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/265257

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-66657f88f820f2b0cab3c1c0a7d8b7f8923af7fb
+4c1e7a083bdea02759cd5d2054038fb8a4a55ec8
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

gcc/go/gofrontend/types.h

@@ -1426,8 +1426,6 @@ class Type
   // The GC symbol for this type.  This starts out as NULL and
   // is filled in as needed.
   Bvariable* gc_symbol_var_;
-  // Whether this type can appear in the heap.
-  bool in_heap_;
 };
 
 // Type hash table operations, treating aliases as identical to the