test-matplotlib: remove sdl

Signed-off-by: Trevor Gamblin <tgamblin@baylibre.com>

Author: threexc

.github/workflows/test-matplotlib.yml

@@ -91,7 +91,6 @@ jobs:
             libffi-dev \
             libgeos-dev \
             libnotify4 \
-            libsdl2-2.0-0 \
             libxkbcommon-x11-0 \
             libxcb-cursor0 \
             libxcb-icccm4 \