Delete MU Plugins & Drop-ins
MU (Must Use) Plugins are located in
/wp-content/mu-plugins/ and are, as the name suggests, must-use plugins that are automatically activated by WP and can't be deactivated via the plugins interface, although if any are used, they are listed in the "Must Use" tab.
Drop-ins are pieces of code found in
/wp-content/ that replace the default, built-in WordPress functionality. Most often used are
advanced-cache.php that implement custom DB and cache functionality. They can't be deactivated via the plugins interface but if any are present are listed in the "Drop-in" tab.
Before deleting anything open the WP Admin Plugins screen and have a look at your MU and drop-in plugins. A lot of managed hosting companies use them and deleting them is not advisable.