Move bending stiffness from plugin to the compiler.

PiperOrigin-RevId: 758677644
Change-Id: I7d5c5140ba6948002bfa4ae9d6b4bf0f6de0e39b

src/user/user_init.c

@@ -238,6 +238,7 @@ void mjs_defaultFlex(mjsFlex* flex) {
   flex->rgba[0] = flex->rgba[1] = flex->rgba[2] = 0.5f;
   flex->rgba[3] = 1.0f;
   flex->thickness = -1;
+  flex->elastic2d = 1;
 }