Removing items from the admin sidebar?

What theme are you using?

Have you tried Appearance >> Widgets >> [open/identify sidebar] >> [open/identify portfolio testimonials modules] >> Delete or drag to Inactive Sidebar for later use.

As that’s a premium theme, you will need to contact the theme author for any theme related questions: http://support.vamtam.com/support/home (they are usually very responsive to support requests).

These support forums remain available for general WordPress and free theme/plugin support.

Regarding sidebar modules in general, you can experiment by dragging widgets into the Inactive Sidebar section and then refreshing your site page to see the change, and then dragging them back into the same sidebar module (in the same position), and saving, if you want to revert the changes (note that you should open each sidebar module and use the sidebar title and widget content to help identify the respective sidebar area in the front-end of the site).

Good luck!