removed extra matplotlib.use call in server.py
removed extra matplotlib.use call in server.py

tbretl/PrairieLearn,mwest1066/PrairieLearn,parasgithub/PrairieLearn,jakebailey/PrairieLearn,tbretl/PrairieLearn,jakebailey/PrairieLearn,parasgithub/PrairieLearn,mwest1066/PrairieLearn,rbessick5/PrairieLearn,mwest1066/PrairieLearn,rbessick5/PrairieLearn,mwest1066/PrairieLearn,mwest1066/PrairieLearn,rbessick5/PrairieLearn,rbessick5/PrairieLearn,rbessick5/PrairieLearn,parasgithub/PrairieLearn,tbretl/PrairieLearn,parasgithub/PrairieLearn,parasgithub/PrairieLearn,tbretl/PrairieLearn,tbretl/PrairieLearn,jakebailey/PrairieLearn,jakebailey/PrairieLearn