Remove a check of VAR_DOMAIN

completion_list_add_symbol checks that the returned symbol has
VAR_DOMAIN, but also checks that its address class is LOC_BLOCK.  The
domain check is redundant -- only functions can possibly be LOC_BLOCK
-- and leaving this in place will cause a regression when combined
with a later patch in this series.  This patch preemptively removes
the redundant check.

gdb/symtab.c

@@ -5631,7 +5631,6 @@ completion_list_add_symbol (completion_tracker &tracker,
      the msymbol name and removes the msymbol name from the completion
      tracker.  */
   if (sym->language () == language_cplus
-      && sym->domain () == VAR_DOMAIN
       && sym->aclass () == LOC_BLOCK)
     {
       /* The call to canonicalize returns the empty string if the input