Remove unused local variable and move local variables into conditional branch

[ken3ypa]

Found dead code, minor enhancement point and fixed them：

• Removed unused local variable association_class_name introduced in commit 9937512664d2338ffe1fe3cd522e84dfbf0cafa6 .
• And relocated local variables referenced solely when association_class.paranoid? is true.
• Streamlined local variable assignments, aligning with the pattern used for association_foreign_key.