Remove all unused using statements from the top of your .cs files.

Leaving unused using statements n the code communicates to the whole team that you don't care about the code quality.

Having perfect code requires a discipline. If you and your fellow developers do not care about unused using statements that are visible at the first sight the very first second when  you open the file, what else does the team not care about?

It is extremely simple to remove unused using statements so do so. No excuses.