epubmaker: fix NameError by unused variable
epubmaker: fix NameError by unused variable

masawada/review,murashitas/review,suzuki/review,suzuki/review,krororo/review,krororo/review,kuad9/review_study,orangain/review,takahashim/review,takahashim/review,orangain/review,masawada/review,murashitas/review,kuad9/review_study,kmuto/review,kmuto/review,kmuto/review,krororo/review