Don't erase empty indices in DWARF reader

The DWARF reader has some code to remove empty indices.  However, I
think this code has been obsolete since some earlier changes to
parallel_for_each.  This patch removes this code.

gdb/dwarf2/read.c

@@ -7170,16 +7170,6 @@ dwarf2_build_psymtabs_hard (dwarf2_per_objfile *per_objfile)
     print_tu_stats (per_objfile);
 
   indexes.push_back (index_storage.release ());
-  /* Remove any NULL entries.  This might happen if parallel-for
-     decides to throttle the number of threads that were used.  */
-  indexes.erase
-    (std::remove_if (indexes.begin (),
-		     indexes.end (),
-		     [] (const std::unique_ptr<cooked_index> &entry)
-		     {
-		       return entry == nullptr;
-		     }),
-     indexes.end ());
   indexes.shrink_to_fit ();
 
   cooked_index_vector *vec = new cooked_index_vector (std::move (indexes));