Plugins deleted from Database and not FTP folder

Hello!

Thank you for taking time to read and answer my question. The website I work on is currently running on v3.5.1 (I experienced way to many crashes when upgrading and 3.5.1 has been the most stable and reliable). Back in 2014 I installed the Super Cache plugin and the CP Multi calendar view. I deactivated them and held onto them in case I needed them again. I forgot about them until I started going through the plugins and deleting the plugins we are not using directly from WordPress. Upon deletion the website would just hang and blank pages displayed. I had to restore a back up to bring the website back. Due to time I left them alone for a while and came back to them.

I noticed they were not displayed in the database but are displayed in the htdoc/wp-content/plugins folder. Yesterday I tried to delete the cp calendar view from the FTP folder and it triggered fatal errors and took the site down. I was able to restore it.

I think the slow loading of our website might be due to these files. Is there a way to delete them from the FTP folder without crashing the website? I really appreciate the help!