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.