The subject is not part of the theme template but will be passed as parameter to the wp_mail(). Also see: https://github.com/arnowelzel/comment-reply-email-notification/blob/1.29.0/classes/CommentReplyEmailNotification.php#L225-L229
If you want the e-mail subject customizable I may add an option for this in future updates.
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This reply was modified 3 years, 4 months ago by
Arno Welzel. Reason: Renamed Github repository
Part of the subject is already customizable actually. There is an option to customize the “New comment reply” part of the subject but the option to customize the entire subject to remove the blog name is not available. Please add this in a future update.
I will add a setting to use custom text for the subject instead of the default subject, since changing the translation to modify text which is used by the plugin is not the correct way to configure a plugin anyway.