gdb: remove subfile::buildsym_compunit field

It is only set, never used.

Change-Id: Ia46ed2f9da243b0ccfc4588c1b57be2a0f3939de
diff --git a/gdb/buildsym.c b/gdb/buildsym.c
index 4718b20..9a9d3cf 100644
--- a/gdb/buildsym.c
+++ b/gdb/buildsym.c
@@ -539,7 +539,6 @@ buildsym_compunit::start_subfile (const char *name)
 
   subfile = XNEW (struct subfile);
   memset (subfile, 0, sizeof (struct subfile));
-  subfile->buildsym_compunit = this;
 
   subfile->next = m_subfiles;
   m_subfiles = subfile;
diff --git a/gdb/buildsym.h b/gdb/buildsym.h
index 4e9aaa4..5f0e023 100644
--- a/gdb/buildsym.h
+++ b/gdb/buildsym.h
@@ -51,8 +51,6 @@ struct subfile
   /* Space for this is malloc'd.  */
   struct linetable *line_vector;
   int line_vector_length;
-  /* The "containing" compunit.  */
-  struct buildsym_compunit *buildsym_compunit;
   enum language language;
   struct symtab *symtab;
 };