Remove unused local variables

These were found by running the specs with --parser=prism in
Ruby 3.4.0-preview1.

lib/sequel/adapters/shared/postgres.rb

@@ -2097,7 +2097,7 @@ def merge_do_nothing_when_not_matched(&block)
       # Support OVERRIDING USER|SYSTEM VALUE for MERGE INSERT.
       def merge_insert(*values, &block)
         h = {:type=>:insert, :values=>values}
-        if override = @opts[:override]
+        if @opts[:override]
           h[:override] = insert_override_sql(String.new)
         end
         _merge_when(h, &block)

lib/sequel/plugins/association_pks.rb

@@ -294,7 +294,7 @@ def convert_pk_array(opts, pks)
           sch = klass.db_schema
 
           if primary_key.is_a?(Array)
-            if (cols = sch.values_at(*klass.primary_key)).all? && (convs = cols.map{|c| c[:type] == :integer}).all?
+            if (cols = sch.values_at(*klass.primary_key)).all? && (cols.map{|c| c[:type] == :integer}).all?
               db = model.db
               pks.map do |cpk|
                 cpk.map do |pk|

lib/sequel/plugins/validate_associated.rb

@@ -53,7 +53,7 @@ def delay_validate_associated_object(reflection, obj)
         def validate_associated_object(reflection, obj)
           return if reflection[:validate] == false
           association = reflection[:name]
-          if (reflection[:type] == :one_to_many || reflection[:type] == :one_to_one) && (key = reflection[:key]).is_a?(Symbol) && !(pk_val = obj.values[key])
+          if (reflection[:type] == :one_to_many || reflection[:type] == :one_to_one) && (key = reflection[:key]).is_a?(Symbol) && !(obj.values[key])
             p_key = pk unless pk.is_a?(Array)
             if p_key
               obj.values[key] = p_key