Warning: fopen, fread and fclose messages after removing formidable

I tested formidable in a multisite set up and decided to not use it. Now when I test sign up for a new site, I get the following warning messages:

Warning: fopen(/home/wpinsta/public_html/wp-content/plugins/formidable/formidable.php) [function.fopen]: failed to open stream: No such file or directory in /home/wpinsta/public_html/wp-includes/functions.php on line 3508

Warning: fread(): supplied argument is not a valid stream resource in /home/wpinsta/public_html/wp-includes/functions.php on line 3511

Warning: fclose(): supplied argument is not a valid stream resource in /home/wpinsta/public_html/wp-includes/functions.php on line 3514

Please help fix this issue. Thanks

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