Don't cache `scope_for_create`

I investigated where `scope_for_create` is reused in tests with the
following code:

```diff
--- a/activerecord/lib/active_record/relation.rb
+++ b/activerecord/lib/active_record/relation.rb
@@ -590,6 +590,10 @@ def where_values_hash(relation_table_name = table_name)
     end

     def scope_for_create
+      if defined?(@scope_for_create) && @scope_for_create
+        puts caller
+        puts "defined"
+      end
       @scope_for_create ||= where_values_hash.merge!(create_with_value.stringify_keys)
     end
```

It was hit only `test_scope_for_create_is_cached`. This means that
`scope_for_create` will not be reused in normal use cases. So we can
remove caching `scope_for_create` to respect changing `where_clause` and
`create_with_value`.

1 files changed, 2 insertions, 2 deletions

diff --git a/activerecord/lib/active_record/relation.rb b/activerecord/lib/active_record/relation.rb
index 23cb1baaed..c5d98e970a 100644
--- a/activerecord/lib/active_record/relation.rb
+++ b/activerecord/lib/active_record/relation.rb
@@ -556,7 +556,7 @@ module ActiveRecord
     end
 
     def reset
-      @to_sql = @scope_for_create = @arel = @loaded = @should_eager_load = nil
+      @to_sql = @arel = @loaded = @should_eager_load = nil
       @records = [].freeze
       @offsets = {}
       self
@@ -590,7 +590,7 @@ module ActiveRecord
     end
 
     def scope_for_create
-      @scope_for_create ||= where_values_hash.merge!(create_with_value.stringify_keys)
+      where_values_hash.merge!(create_with_value.stringify_keys)
     end
 
     # Returns true if relation needs eager loading.