Remove dead :unions_with_bind render branch

The non-optional `:unions_with_bind` branch in `Query#to_s` was
unreachable: its DSL writer (`union_with_bind_as`) was dropped during an
earlier reset-to-upstream + manual reapply, leaving the render branch
orphaned with no way to populate `options[:unions_with_bind]`.

Confirmed unused across all active downstream lineages (ncbo/goo,
ontoportal/goo, and the AgroPortal/LIRMM `ontoportal-lirmm-development`
branch) — every consumer calls only `optional_union_with_bind_as`.

This is a pure dead-code removal; query output is unchanged. The live
`:optional_unions_with_bind` path and the shared `add_union_with_bind`
helper it relies on are retained.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: alexskr

lib/sparql/client/query.rb

@@ -843,12 +843,6 @@ def to_s
 
 
 
-      if options[:unions_with_bind]
-        buffer.pop # remove } of where
-        buffer << add_union_with_bind(options[:unions_with_bind])
-        buffer << '}'
-      end
-
       if options[:optional_unions_with_bind] && !options[:optional_unions_with_bind].empty?
         buffer.pop # remove } of where
         buffer << 'OPTIONAL {'