Remove sdflib plugin and move interpolation to engine_collision_sdf.

Before:
```
 Simulation time      : 1.71 s
 Steps per second     : 5854
 Realtime factor      : 11.71 x
 Time per step        : 170.8 µs

 Newton iters / step  : 2.19
 Contacts / step      : 3.45
 Constraints / step   : 13.79
 Degrees of freedom   : 12
 Dynamic memory usage : 0.2% of 14M
```

After:
```
 Simulation time      : 1.41 s
 Steps per second     : 7093
 Realtime factor      : 14.19 x
 Time per step        : 141.0 µs

 Newton iters / step  : 2.19
 Contacts / step      : 3.45
 Constraints / step   : 13.79
 Degrees of freedom   : 12
 Dynamic memory usage : 0.2% of 14M
```

PiperOrigin-RevId: 781087896
Change-Id: Iaf69dccc5e95af5bac862344683c74f962162186

src/user/user_mesh.cc

@@ -1551,8 +1551,7 @@ void mjCMesh::Process() {
     octree_.CreateOctree(aamm);
 
     // compute sdf coefficients
-    // TODO: only check !plugin.active once sdflib is removed
-    if (plugin.active && *plugin.name == "sdf") {
+    if (!plugin.active) {
       tmd::TriangleMeshDistance sdf(vert_.data(), nvert(), face_.data(), nface());
 
       // TODO: do not evaluate the SDF multiple times at the same vertex